1. they are complex. but they come with comprehensive teaching/training tools, even if admittedly some are more mundane than others. if you truly dont want to even have to try or put in effort, play Strive and possibly troon out in the process. otherwise rely on the abundance of lessons learned from other players posted online in something available that gets sequels often like street fighter, rather than antagonizing over forward-facingly dense things you cant even participate in like Tekken. or start small with free less commital options to build up your confidence and recognition of what they ask of you.
2. Typically yes, on a macroscale compared to other genres they die sooner than other games, especially legacy titles as they get held together in smaller spaces be it discord or general threads or whatever is most apt these days, but as newer games get sunsetted they often have features transferable to one another as new ones. If you want a blank slate, 2XKO is also releasing soon, made by Riot Games, thus potentially capturing a pre-established larger playerbase as well. And again, theres fightcade too for even older games. But still even for dead games it isnt much of a trifle to find someone to match with.
3. Because you have abundance of options. it just matters that youve applied yourself honestly and earnestly even if it sucks to suck its up to you to discern at what point your investments were either of value or a waste. Doesnt have to be a fighting game. Tetris TGM4 recently dropped as a singleplayer game to gitgud in and overcome walls, but theres also multiplayer options like Puyo Puyo Tetris 2. Another poster listed climbing the ladder in an ARPG which are arguably even more complex thn fighters, shifting the mechanical load into a logical load. You can even do niche shit like Dr Robotniks Ring Racers. The joy of gaming are the variety of options and expressions, even when you hit that wall in your journey of “there are no games“