You're assuming that if Apple and Google forcibly lost the ability to profit off app sales through the stores, they would continue to offer access to their commercially licensed software development platforms for free. If Google and Apple are forced to give away what they used to sell, they will simply change the mechanism by which they extract payment and developers will need to prepare for e.g. licence fees to access platform features.

And to some extent, controversially, I ask why not? If Epic can take a percentage of revenues if you use Unreal Engine, Apple can rejigger the way they structure their libraries and demand a percentage of revenues from apps which use Metal. I realise that there are intellectually satisfying arguments for why UE and Metal shouldn't be treated the same, but I'm not so sure it's so easy to disentangle them in a legally satisfying way.