Not a storefront per se, but I think Blizzard's Launcher that includes most of their newer games (as well as Destiny 2!) is a good example of one that's gone from nothing to pretty-darn-good while the Steam client has been relatively stagnant.
That said I think the Valve hating in this thread is... overblown? DOTA2 and CS:GO are some of the most played games period outside of Fortnite, LOL and PUBG. HN has been complaining about Valve not releasing a new game for years but why release a new game when you have multiple, popular, heavily played titles? All the complaints seem strange coming from a site where many of the users work for SaaS companies that don't follow a business model that requires releasing an entirely new product every year or two.