It's hardly about the quality of the client. It's much more about force of habit.
The Steam client, whatever you or I might think of it, offers a lot of people a lot of features they genuinely love. In any event, whatever you want to ascribe Steam loyalty to, the point remains the same. Epic seem to be playing the "get people in and then turn them into real customers" game, but I'm not sure how they plan to actually do the second part. I don't think your theory of it all being about new customers holds water, since the exclusives game is all about making people switch from other platforms, and since they've been very open about wanting to knock Steam down a few pegs.