In the beginning it looked like no-brainer – of course to play.
I will see better simple threats/combinations, try openings, etc.

OK, apparently it helped, in 9 months I went from “C” to “B”, began to make less gross errors, win more (got some killer instinct), but …
My rating hit plateau, I got 3 endgames out of 5 games in the last tournament – won only one, had chances to win in 2 others, got only 0.5. Of course, loss was under time pressure, still … did I know how to win? Do they teach it on FICS?

You play game after game; most of them are decided by blunder(s)/simple tactics,
you do not remember any of them after some time.
Yeah, scarcely you find a nice combination or play a good endgame, so you feel proud of yourself. Quite a few people play weird openings (meaning not known to mankind), so you are very lucky when you get to play your favorite ones.
Their favorite – Giuoco Pianissimo bores me to death.

When I feel “sharp”, I can win 8-9 out of 10 (with same/lower rated players),
when not – drop 4-5 in a row, when not concentrated at all – lose to patzers and get
rating down by 13-15 points at once ( my stomach almost hurts after that).

You lose a few games, it’s like playing cards and losing money, you want revenge
( and lose more). You stay late, sleep 4 hours, then go to work. I am accustomed to sleep not much, still doesn’t look right.

So, after playing ~2500 blitz/standard games in a year, what next?

Play with much higher rated people? Yeah, they play “good” openings, and know other things well too. So, they will crush me time after time.
Is it good for me? I am not sure, even I decide not to care about rating at all.
I played OTB with people rated much higher then me, and though I resisted
more than 30 moves every time, the game was always going one way.
Maybe I learn something, but my ego suffers.
So, play with longer time control? I like OTB long controls, but online it looks boring/too demanding (your free time is not unlimited).

Do not play at all? There should be some use …