I'm not saying this is ideal or practical, but I thought about this when I started playing on a community-run City of Heroes server.

It's an MMO which was shuttered in 2012 despite a very devoted fanbase. People had been clamoring for any chance to play the thing again for years. As an example of this devotion people were calling it a big deal when a program became available that does nothing but allow players to walk around the first hub world and chat together. It was all they could do.

Then word breaks that there had been a private server running in secret the whole time based on source code that had secretly traded hands a long time ago.

Then, regardless of the moral implications, someone went ahead and leaked the source code. Because of that the general public finally got to play the game for the first time in over six years. Apparently it has 100,000 registrants on the most popular server alone, which I find ridiculous for a game which has resurfaced out of nowhere with no real advertising and whose publisher never intended it to return. Imagine all the revenue they're missing out on.

(Of course, it's only popular today because it was popular in 2012 when it was subscription-based.)

From my limited understanding there doesn't seem to be massive feature updates like new maps or raids when Paragon was developing it. Instead there are minor quality of life perks like making the paid upgrades purchasable with in-game currency or tweaking formulas to align with the playerbase's wants. You don't even have pay for anything to keep playing individually; the server is funded by volunteers with donations. The monthly target seems to be about $5,000, most of which goes to hosting fees. Since enough interested people keep paying, the server stays up. And since the source code is now available there's nothing precluding another party from starting a different server if things go under. All to be able to keep playing the game.

It goes to show that people are happy just being able to play the thing at all. There's no need for large salaries to keep developing content and justify monitization. The code is worked on by a devoted team of people with a very big passion for the former-product, and the devs and users are part of the same community.

In a perfect world this is probably how an MMO could be sustained for years to come. And it's possible primarily because the source is available.

Of course, the only reason any of this is possible is because the parties involved blatantly violated copyright by posting the source code and using proprietary assets like textures and music - but this doesn't matter because, for some reason, nobody cares. Yet. Maybe it's because NCSoft figures they're not competing with anyone since they were the ones to shut it down years ago so there's no harm in letting it stay open. Maybe they just got lucky NCSoft wasn't disingenuous and that they weren't slapped with a draconian takedown demand for a very popular game that would otherwise never be playable by anyone until the end of time. But for some reason, the people with the most interest in where all the development capital went just let it slide - for now.

Also this differs from a lot of open source projects since it's technically illegitimate, none of the code is under an open source license, and the work being done is fairly minor. Basically everyone got a head-start by being able to reuse Paragon's code back when the people writing it were salaried and experts in this kind of thing so the complicated parts of the MMO architecture are already finished and everyone can focus on the small quality of life things.