If there is no mechanical override, whatever computer is controlling core functions had better fucking be isolated and running code that has been through "industrial grade" review processes and such. If everything else dies, that computer should still work. It should even have its own little local backup power of some kind, or be able to pull power from something other than the main vehicle power system. I'd add that it also should not permit OTA software upgrades at all, and its connection to the higher level systems should be least-privilege.
There really should be a manual override, but in the absence of one that is the only acceptable design for a fly by wire system.
I just opted for a 2022 Nissan Leaf over a Tesla Model 3. Cost was a major factor (and I'm not a frequent road tripper so the faster Tesla recharge was less critical), but so was this kind of stuff. The Tesla is far sexier with a better design and has a lot of desirable features, but I am glad that I did.
As near as I can tell the Leaf is designed like a normal car. It seems electronically controlled (I have not looked for mechanical overrides but I might), but I get the strong sense that the computer and electronics controlling that are not the same as what's running the infotainment system or even the lane keeping (it has level 2 self-drive). They are probably discrete systems. It also has a 12V lead-acid battery that can power all the electronics if the main power system totally eats itself, which is not an impossible scenario given that an EV's main power system is some hard-core high voltage electronics. Nissan can also align panels.
I really really really want to like Tesla, but this is beyond insane.
I sincerely hope that SpaceX avionics are not designed this way. They probably are not because NASA would not let its astronauts fly in it otherwise... different market with much more informed customers.