Here's a quick breakdown: Linux is not an iPhone. One is hardware, and another is software.
Apple has every right to build a hardware product with a software platform, but controlling the distribution of software is an immediate red flag. Conflating security and limitation is meaningless since iOS already has the means to isolate software a-la MacOS. Worst-case scenario, Apple can add a "Christian helicopter parent" toggle for you in the Settings app and let you opt-in to whatever you're enjoying now.
> Even Steam has some arcane filters that eventually allow porn content to be visible in the store.
Porn doesn't violate Steam's TOS. As long as your account is registered as under-18, it won't even be possible to see it.
> But despite all that, why can't Linux devices serve the "control my devices" need.
It can. On the newest Apple Silicon machines, I even hear it's excellent. That being said, Apple needs to unlock the bootloader of the iPhone/iPad to give us that option, so as-is our only choice is paying them 30% every time we make a transaction. It's so nefarious that I won't let anyone say it's 'serendipitous with moderation' or whatever you're trying to argue here. It's bad-faith and disregards the reason why we're talking about locked-down platforms in the first place.