Eh you should play the game because you're interested in improving at it. Not because of the reasons you listed. Under your logic, SC2 would be favorable over games like Quake, a majority of fighters, and osu even though all the games listed require a lot of individual skill.
Anyways, SC2 is actually doing fine. Since they went F2P, there's been an uptick in viewercount and prizepools. 2017 and 2018 had the biggest prizepools in Starcraft's 20 year history.
If you just want the game with the biggest casual and competitive playerbase then there's CSGO, LoL, and DotA. Hell, SC2 #5 in esports prizepools and there's a pretty massive drop for the #6 game.