Soundpaint, Sampletank, LABS, Best Service Engine, UVI Falcon, SINEfactory, Decent Sampler just to name a few

None of those are standard and open except for Decent and UVI which does have third party developers albiet not many. Anecdotally the only dev I know on UVI is acoustic samples? Since Virharmonic moved off of UVI onto their own powered by gorilla

The rest you listed, Soundpaint, Engine, etc. are company specific proprietary samplers. Their only market share is the market share of their own company

A lot of these aren't even samplers?

SINEfactory is just OT's free collection of instruments. Labs is not even a free collection either anymore, Spitfire has done some strange subscription or monetization wrapper around it (and fyi labs used to be on Kontakt).