The issue persists in that there are WAAAAY too many games on the market now, and the shelf life is extremely short. They need to squeeze what they can out of players, easy learning curve, special abilities, paid DLCs, and the like.
That being said, I believe there are two things that hurt the Quake community:
1. Not capitalizing on the game's amazing run during the early 2000's when the most popular players went head to head and there were no games on the market more exciting to watch from a fan perspective.
2. The competition.. I see guys in our Quake chan on Discord playing games like 'PUBG' or 'H1Z1'.
Most games now are actually only half a version of the full game on release, and then you just pay for DLCs throughout to get the full thing.
I've enjoyed BF1, but the lack of community in PS4 console games is just brutal.. some games are a bit better (like I have friends who are in clans in games like Destiny or Overwatch).
Nevertheless, this is a special place, the guys still play Q1 on discord through the community and it's a real niche thing, and I don't think that can ever be duplicated in future games as well.
Communities are too large, players are mostly anonymous and from all over the globe.
There is real nostalgia to certain older games now.. games like WoW are among them as Baker noted.
To the point above from Baker about gamer populations for various games. A big part of that which I think you may have forgotten to mention is the eSports scene.
For example SC2 has been around since 2010, it was in steady decline because the game itself was weak. However, the latest expansion 'Legacy of the void' managed to get it right and capture some of what made Broodwar special.. I've kept up with the SC2 Reddit threads and they've had consistent player growth since the expansion came out; many older players coming back to play it. Moreover, the eSports scene in Korea for SC2 has also ramped up.. I watch the GSL and SSL tournaments weekly in Korea with English casting (the casters are hilarious and people love to come back and listen to them - they're expats well known in Korea for SC - SC2 commentating who go by the name of Tasteless and Artosis - they even have a combined name 'Tastosis' for their symbiosis and humor while casting.. why is this important? Well they have identifiable faces, guys can relate as a community and come back to see their favorite guys.. those guys also come and join up the reddit threads and chat about the game..
Most of the older games you see surviving (aside from TF2) have a real strong community but also have an eSports scene to aspire to and watch (CS GO for example).
Quake used to be the hub for eSports, Quake 3 was THE most popular game at one point in the early 2000's when Fatality and Zero4 were going at it. Instead of ramping that up to the next level, they made a mistake by botching Quake 4 badly and not have it resemble Quake 3 properly. That's where things started to derail.
People used to make amazing Q3 RJ videos, highlight compilations of the top guys from Europe playing at LANs for big prizes, and the game was booming... they didn't handle the decline properly and now it's playing catch up ever since.. problem is the community is sparse and guys have moved on from gaming (I know myself and many quakers who now have families or have gotten married and moved away from gaming entirely, so this is in large part why Quake is having a hard time capturing that audience and needs to appeal to a new younger demographic to catch them).