You can agree or disagree with that mission, but it is what it is.
ł€æßðđŧŋ«»¢“”←ħn↓ĸ→ø
Those are input from just pressing ALT(right) + <SOME LETTER KEY>
The € is ALTgr + e. ¢ is ALTgr + SHIFT + e.
Arabic key layout is different (which fine, OS X uses something non-standard anyways), but makes it absolutely impossible to customize it. Text boxes frequently jumble themselves up with left/right mixed input, while OS X handles this as expected.
Chinese input is slightly better, as you don't have to deal with order wonkiness. But pinyin input is halfhearted at best, and requires a bunch of qol changes.
This is how it has always worked (I use a Latin-American keyboard layout). And I think it's the best of both worlds.
If you wanted and your font supports it you could have: ṕẃéŕý (accent key + character key).
All above without touching the AltGr key.
Too often I want to use the proper spelling of someone’s name and I run off to Google to get the symbol.
you can also add languages and use the on screen keyboard in said languages
I don't see why introducing a fourth one would help?
The main reason you can't simply flick a global switch between those three but have to do it per app seems to be the license of GNU readline.
Seriously, though: emacs has several terminal emulators built in like ansi-term, comint, and eshell.
Windows/linux still defaults to many OS commands being in the ctrl space (like ctrl-c for copy, not windows-c) which causes clobbering in the terminal, forcing people to use ctrl-shift-c, for instance.