If you look at
https://stats.xonotic.org/ you will see that every day 500 people play the game. But of course not all at the same time, each person only plays 15-30 minutes. And some people only play on the weekend, or every other day etc. So if you crunch some numbers, you will find that the entire active playerbase is in the thousands of players. And you have like 10-40 people connected to servers at any point in time, which is a lot if you think about it. Xonotic is by far the biggest and most popular FOSS shooter. For whatever reason, few people in the US are playing though. So if you are from the US it might be harder, since player count is very low at 3-8 UST or so.
Granted, it is not 1999 anymore where ther were like 6 good games in store shelves and everyone was just playing Quake. Nowadays you have tens of thousands of games released each year, and Xonotic is only in some "small" corner, like all the other great games that are not released by mega game studios, or simply the product of some far-out chance of going viral.
But yeah, if the playerbase were to shrink too much, then you had more and more hours a day where you can't really find someone to instantly play with. So people will move to other games, and then it quickly becomes a vicious cycle. That's how all the other ArenaFPS died too.
All forums are sort of dead nowadays, people have moved to Discord and Matrix and such.
Concerning development, this is a hard topic to tackle. On the one hand, there is still a lot being done to keep the game up to date and do bug fixes and lots of small things. But on the other hand the game is stuck with a really really obsolete engine. And this is why you will never see any more ground-breaking new things or new game-mechanics added. Or stuff that could make the game more viable on Android, Steam, social media, etc. The game is what it is. And in terms of what can realistically be done with the engine, the dev team is doing an excellent job. We are at a point now where Xonotic could be easily "superseded" and taken to the next level with a modern engine. But no one has really bothered yet to wrestle with that reality. Truth is, competition with commercial games is too tough, and ArenaFPS are sort of outdated. So to really overcome this, it would take a lot of polishing, innovative game design and investment. The market is totally overflooded with games, so having yet another game has lost it's importance. Shame though.