There's no evidence that developers/publishers would be charging less if they didn't have to pay Apple 30%. And there's actually evidence right here to the contrary: Fortnite is charging only 20% less for direct payments, meaning they're pocketing the 10% difference.
I think the evidence points to developers and publishers wanting to raise prices due to increasing development costs. That's the line that's being used for $70 retail prices for games being teased for the PS5 and Xbox Series X. I'm sure it wouldn't take Epic very long to use that same line for raising the price for Fortnite even if they did pay less for Apple's store cut, i.e., "we passed along the savings for as long as we could, but our dedication to quality and rising development costs require a new pricing structure".