This is one of the strongest arguments for why ordinary users need the ability to do this. Not only because they would actually benefit from it (e.g. better ad blockers), but because things like that have a network effect that you can't get from the four people who root own their iPhone with a PDF being the only ones able to install a third party browser engine.

Retro game emulators were allowed only 2y ago and bold retro. JIT is still not allowed so they are restricted in being run in interpreted mode. Also, neither is allowed executing arbitrary arm64 binaries. And, of course, no hypervisor access. The Linux-using term (iSH) works by utilizing a x86 emulator / compatibility layer (basically translates x86 binaries and maps Linux calls to native ones). In the sole VM host available, UTM SE, the "SE" means "slow edition". Now jailbreak and you've full, JIT-capable, UTM (QEMU) and NewTerm (kinda Termux for iOS). Hypervisor access still not possible though.