> What's the issue with that?

Processes (unlike threads) are for memory protection. There's no relation between the number of processes and battery life.

> What's the advantage when all the devices, your apps are supposed to be running on, share the same OS, platform, ABI etc? What's the point when the hardware is already limited in scope compared to alternative platforms

What if you want your users to be allowed to write code?

> Again, why would you want some random developer be able to download extra code?

Again, to allow users to share code (e.g. allow game mods).

> What prevents them from not misusing it for nefarious purposes?

The same thing that prevents them from having native code doing nefarious stuff: nothing.

> what prevents a third-party from compromising the developer's "update" servers and pushing malicious code?

Nothing, just like an native app can be compromised. That's why Native Apps are in a different process, and that's (one of the reasons) why Native Apps should be able to create their own processes (e.g. run a game mod in a separate process from the rest of the game).

> This is from a bygone era.

Okay, but this may happen again.