One thing I'm sure of, though, is that Steam is way too powerful. They have far too much control over PC gaming and that needs to end. Valve can be crappy at times and there are too many people who get defensive over them like they're one of the console manufacturers instead of calling them out for it.
The thing is, is that Valve have no pwoer, in forcing devs to come to their store. Devs go to their store ebcause it has the most people using and buying from it.
The competition out there is doing hardly anything to lure devs to their stores. MS isn't doing much of anything to court devs to release a plethora of titles, both indie and AAA to their store (unless it's what they own). Ubisoft only sells a tiny handful of games (again, most of what they own), the same goes for origin. Battle.net sells only games that Blizzard/Activision own, as well as Bethesda.net.
The only store out there, that actually sells multiple types of games that Steam has is GoG, and they primarily focus on retro games being fixed for modern OS's. They sell some indie titles, but they pick and choose a few to sell. The competition out there simply does not do enough to court devs to their stores. Valve is the only one out there that basically has the gates open for the majority of devs to sell from that storefront. They are even the store that fully supports the modding scene. GoG doesn't even have their own version of Steam workshop.
You cannot simply force a company to stop everything, and tell other devs to go to the other storefronts. That's not how it works. The other storefronts have to really put in 110% of the effort needed to court those devs and studios to their stores.
I think Valve are shit when it comes to quality control for their store, but I'll give them a pat on the back for fully supporting the modding scene. The other storefronts are seemingly against modding games (GoG isn't against modding, but they don't support it the way Steam does).