Plenty of games are playable through multiple launchers, R6S comes to mind off the bat for me. I know it allows you to launch through Steam and essentially just launches UPlay before the game opens up. As far as I know those systems don't bug anyone because you are still playing with the whole PC population. There was a CoD (Infinite Warfare maybe?) recently that launched on MS Store as well as Steam and it didn't allow any crossplay between Steam or Xbox. It's population was so low that they were giving refunds out for the people that requested it. I think it's a non issue to sell on other platforms as long as they play together and selling your product on Steam well undoubtedly increase your sales number just by virtue of being the largest PC launcher.
Allegiance skins for sale in game.