I don’t think the number of games matter. Most games are just not valuable to either Steam, Epic or even the developer (in terms of monetary gain). Most will never see a profit. Most are money sinks for both the developers and for Steam. We already know this.
Like an angel investor, Valve only needs a handful
of games to succeed on steam for it to be good make up the costs of all the rest. For the longest time, steam was trying to limit the number of indie games on the market, so that it would have to deal with the 99% that just take up room.
So, for the sake of argument you have 100 games coming out from indie developers. For the sake of argument, maybe 10 of them will be good enough to turn a profit of any significance. So, those 10 games will probably pay for the costs of hosting the other 90 and then some.
Now, Epic tries to game the system by figure out what those 10 are and maybe make as offer on 8 of them and maybe 10 or so of the sucky games. Well, even if they snag 5 of the good ones and all the crappy ones, that leaves Steam with 5 games that have to support other 80 crappy games left over, while Epic has 5 good games and now 10 bad ones.
Steam doesn’t care about the 80 crappy games it has, by it sure as hell cares about the 5 good games that it lost to Epic. And since Steam has a pretty open system, it has a tougher time figuring out which of the 100 games will be good before Epic comes in, so in order to keep them on its payroll, it needs to make Epics offer less beneficial by offering better deals to all 100 games. The model that Steam uses doesn’t allow it to pick and choose winners and losers like Epic can (at this point).
Anyway, that’s how I foresee Epics strategy having an outsized effect on Steam because they are trying to target Steams money makers.
Now, Steam doesn’t have offer everyone a better deal. It’s already gamed the system by offer everyone that sells more then a certain amount of units a better deal. Certainly, they can keep doing that instead, and developers that are confident in their game might decide to stick with Steam because of that. But, I think they would still need to lower that amount to make it tempting, which is still a net gain to developers.