The best case to expect is an average Spotify user listens to about 500 songs a month (25 hours) and has an ARPU of about $5 (https://techcrunch.com/2019/07/31/spotify-108-million/). There cannot possibly be more than $0.01 (1c) per play available to pay the artists. Spotify paying out 37% of that actually looks pretty good.
That's as high as the users will go, it seems; the equilibrium point against piracy or other music services.
And who buys albums nowadays? I made some digital download purchases, but now they don't make much sense.
Smaller artists get paid less on Spotify but bigger artists get paid more (not sure about the details)
Per play, yes, not in total. But they compensate by getting artists a lot more plays than others.