Their sales used to give you a reusable $10 off coupon. That didn't change the amount the companies got when someone bought their game. It only changed how much they paid. When one of the Witcher games had that coupon applied to it, the developer got pissed off and changed the price of the game so that it was a cent or two below the threshold to activate the coupon, and then fans of the dev were excusing it claiming that they couldn't let the price be lower because it would 'devalue' the game.

if a game was $30 on Steam and $25 on Epic (as a regular price), or some other service, you'd undoubtedly hear the same rhetoric.

Epic's cut is 12% not 30%. They also waive the 5% royalty fee over $1 million for sales on the Epic Store if you use Unreal. Epic doesn't control the prices. Devs set the prices. They leave the price the same on Epic so that they can actually get a little more for each sale.

What the should do on a $60 game though is to set the price at like $56 on Epic, it would encourage people to save a couple bucks there, while still getting them more than steam after the cuts.