Having gone on at length about the problems with Epic's storefront, and with their profoundly consumer-dismissive approach to... well... everything, it's only fair to spend some time and words on the issues that Steam actually does have, which GabeN should probably attend to. Because after 16 years in service, it's fair to say that Steam's pipes have some rust and corrosion on them, and really could use a good cleaning.
Gabe! Buddy! My nearest and most excellent friend (that I've never met in person, and who doesn't know me from Adam, but whatevs don't@me)! I have some advice for you; a five-step process that will clear a up a whole bunch of that embarrassing clutter that's causing so much agita, lately. Take these ideas, and use them in all good health.
Step 1: Archive stale content
Any game that hasn't been purchased, wish-listed, or updated by its developer in the last two years isn't inventory; it's dead weight. Get rid of it. Put the game into an "archival" mode, which will allow customers who've already purchased the game to still download/install it, but stop allowing new purchases of the game, and stop displaying the game in search results. If you really want to, send an email notice to anyone who wishlisted the game (more than two years ago, natch) a month ahead of time, letting them know it's time to piss or get off the pot, but after that, get out of the business of selling that stale content.
You can allow developers to re-list a game provided they have an updated version ready to go, but that should be handled case-by-case by a human, and not by an automated process, to verify that changes have been made outside of the version number. You should also charge a fee for this service, to pay for the human involved, and your server costs.
Step 2: Remove terrible content
Gabe, I get it; you're running an open marketplace, and your entire philosophy is that you (i.e. Steam) don't want to be in the position of judging the tastes of your customers, or the developers who make the games they're buying. Short of games which are obviously just trying to get you (i.e Steam) into legal/political trouble, you'd rather get out of the way, and let the market decide the fate of the games that live there. Well and good; so start listening to that market when a consensus has clearly been reached.
Any game that has a refund rate of 50% or more, which is to say more customers returning than keeping it, is a terrible game. That's not your judgement, or anyone else's subjective judgement; it's the verdict of the market. If more people are returning a game than are enjoying it, then that game is probably costing you money to host, if only in all those transaction fees; so archive it.
One caveat here, of course, is that you'll need a guard rail to prevent the refund equivalent of review-bombing; if you see a huge spike of people buying and then refunding a game immediately, then you'll probably want to look into what the current state of the discourse is surrounding that game, its subject matter, its developers, etc., so that your system isn't easily abused. That done, though... seriously, kill these shit games with fire.
Unlike case #1, games that are archived should not be offered to people who've already wishlisted them. Because those people really aren't interested; you're not doing them a service by taking and refunding their money, and wasting their time in the process.
Again, allow de-listed games' devs to appeal, but have a human judge those appeals case-by-case, and require the developer to proved that they've actually fixed the damn game
before agreeing to re-list; refuse any requests to simply re-list the game as-is, on the
grounds that its terribleness was somehow a feature of some kind. The
market has already spoken, remember, and terribleness is not a feature
that anyone really wants to pay money for.
Step 3: Begin suspending inactive &/or terrible developers
If x% (or more) of a developer's titles have been archived, then they're not worth doing business with, and should be suspended. Allow them to appeal the suspension, but have those appeals judged by a human on a case-by-case basis, and charge a fee (to cover the cost of those humans) up front to begin the appeal process. Note that "x" can be any value that makes sense, here: half, two-thirds, three-quarters, etc. You can assign different weights to archived games based on their archival reason, too; games that were archived for lack of activity should each count as one game, but games archived for terrible quality should each count as a game-and-a-half, or as two games.
I'd recommend that you also suspend any developer doing business under multiple different names, and other such scammers, but I think you're already doing some of that; just do more of it. Hire people whose whole job is to clear the snake-filled underbrush, and then give them axes. Your community will only improve as a result.
Step 4: Remove the other clutter
Some of this is stuff that I think you're already working on, but I still want to mention it here. Search results should only display DLC for games that the searcher already owns; all other DLC should be hidden by default. Searching for Mortal Kombat should show me a list of MK games, and not three pages of DLC for the games that I don't own yet, and can't make heads or tails of anyway; games like Senren Kagura are even worse. By all means, sell the games; I judge neither those buyers, nor the sellers, nor you. But clearing this clutter away from Steam's search pages will go a long way to improving discoverability for the games which you let stay.
Step 5: Rinse and repeat
Bear in mind that these steps are not one-and-done tasks, but will need to be done as an ongoing part of maintaining your service. It allows you to deliver on your chosen mission of connecting game developers with the people who might love their games, no matter how niche, but will help you do all of it better... and, in all likelihood, improve the efficiency and performance of your servers, too.
You can even supplement this with another level of clearance: giving Steam store credit for any archived game which a user hasn't played in over two years. The amount can be nominal ($5 or the regional equivalent each can still pay for a good Steam sale deal), but convincing players who have stale games in their libraries to recycle that server space should be worth at least a couple of bucks to you. Once an archived game no longer needs to be kept for potential downloading, you can remove it from your servers entirely.
There is one big problem with the whole approach I've outlined here: it doesn't do much to preserve older games for history/posterity. Let's face it, though; preserving the historical record of video gaming really isn't what Steam is built for, and your "lite DRM" approach to digital ownership will never be the equal of a service like GOG/Galaxy when it comes to game preservation. So, rather than trying to swim against the current of what your service is clearly built to do, partner with CD Projekt/GOG instead; let them know, in advance, what games you're archiving, let them select which of those games they want to pursue as potential preservation projects, and provide them with whatever resources they need from your servers (except for your customers' information, of course) to facilitate the process.
There you are, Gabe: a five-step process which will clear the clutter from Steam, and cull the collection down to something more manageable. You're welcome.