Valve pretty much created the PC gaming market as we know it today. Before them you could buy the games from retail, pretty much the same as in a console with the particularity that there was no unified ecosystem for updates and MP playing.
As developers:
You had to have a site or something similar to deploy updates
It was easier to hack/crack (No active DRM or it quickly became obsolete)
Had to take the risk of subcontracting an online platform like gamespy or something similar
No statistics other than what your own code could pick up (if you had the infrastructure to receive that info back)
Had to rely on traditional marketing to get eyes on your product (indie developers can't afford a 2 page spread in EGM)
As gamers:
You had to be able to find the game you want in retail. Easier for newer games, difficult for older titles or niche games.
You had to use third party platforms to try to have a community such as X-Fire
You had to download updates manually, files could be difficult to find, connections were slower and sometimes a 500 mb corrupted file could mess up your weekend
No sales
PC gaming outside of the main markets was pretty much piracy based (like in Mexico)
So Valve destroying the PC gaming market? Don't think so. Even if they did, they pretty much created it so I guess it could be theirs to destroy. Just as if Sony decided no to support PSN any longer (for whatever reason).
More stores equals more competition but it's only good for us as consumers if they are offering something better than the rest. As far as I know the only thing Epic is offering that is objectively better than Valve's offer is a bigger cut for devs which is awsome. However, as consumers we don't even have feature parity and savings are not being passed on to us.
So right now, as a gamer, why should I care for EGS? (other than not liking that I have to choose between waiting for an exclusivity deal to be over or playing on the technically inferior platform)