Keep in mind that you are literally saying (well, implying) that being fine with the developer settling for "merely" the standard 70% cut out of every sale is equivalent to starving them to death.
A bit hyperbolic, honestly.
Especially since 88% of X sales isn't in any way better than 70% of X sales multiplied for N times.
Don't falsely equivocate.
I didn't say it was equivalent to starving them. I said, "helps kill off", as in, the callous opinion you have expressed is a factor that if widespread, can certainly contribute to making developers go out of business. I think that's a reasonable conclusion.
In contrast, if you passionately supported a particular developer and wanted to help them succeed, one option could be to buy their game from a launcher that gives them more money. This is why some people buy new so that the developer gets the money. This is why I sometimes buy at full price, if it's a developer I really think deserves it.
Consider that if you do so, and the Epic store takes off, that could encourage Valve to actually give developers a higher cut. Epic had a very developer-positive move a while back in which they retroactively gave a higher dev cut. Doing something like that retroactively is almost unheard of.
If competition ends up making developers in general end up treated better by both companies, your decision has helped more than just one company. In fact, it might even help people like me who have tons of Steam games since its inception, and who mostly game on Steam, because it might get Valve to actually improve such features. My friends list still has an interior search compared to before the most recent update, for example, which is sort of ludicrous.
Note that I have lots of Steam games, almost always buy on Steam, like several Valve games, advocate the Steam controller's use, and like Linux.
And yet, I still express such opinions. I even downloaded Origin of all things. Why? Because I passionately wanted to support the team behind one of my favorite games ever. Because there are people behind games. Because it's not a middle school console war where everyone has a team, and we trash anything different.
