I wonder how much I paid over the last 15 years for the 198 games in my Steam library. Not that much, I suspect between all the Humble Bundles and steam sales of yore. Nevertheless I was pissed when I had to get Origin in order to even play Mass Effect 3, and I never even considered the epic store, so I think the theory of library investment is sound. Steam has a good head start on a lot of us.
Everybody hated Steam when it was new too and with the frequent Epic game giveaways people will eventually have large catalogs of Epic games they grabbed on a lark. Between that and a number of highly desired sequels being exclusive to the platform I can't see why they wouldn't be able to eventually make inroads.