Is being able to replace software on the devices I own not in "the realm of software" somehow?

"Sure, you can have the sources, you just can't use them on your own devices because the vendor that shipped it has decided to bar you from doing that with a 2048-bit RSA key" just feels like GPL was upheld in letter, but not in spirit.

Yeah, it is -- you're asking for restrictions on pieces of hardware unrelated to the original software other than the fact that someone decided to install it on there.

How would you feel if a piece of hardware came with a license prohibiting software developers from using encryption to secure their systems?

The root of the issue here is that phone hardware landscape is effectively a duopoly. It is an antitrust issue. Trying to use software licenses to do this 1) won't be effective because the duopoly will never use them, and 2) is like going around your ass to get to your elbow. Even if it did work it wouldn't get to the root of the issue. The law needs to fix the fact that almost all phones on the planet are controlled either directly or indirectly by two companies.