Not OP but to your last question I think there's proven danger in the "too familiar, too easy" zone, i.e. most car accidents for instance tend to happen in places you know pretty well — hence why you may get surprised when things happen out of the ordinary.
Whether it's an illusion of safety, a letdown of attention, the general idea is that humans should never trust that things will go well when there's real probability that they don't. I think it's not in the amount of automation, as you explained well, but rather in focusing users on the critical parts that they should watch out for — and there clearly automating helps us remove the unimportant from the equation, and also make us more responsive, more accurate for the important parts. But it's a lot of great UX, and that's one field where e.g. the military is usually great but commercial companies are abysmal if they can get away with it (read: sell enough to justify not spending a dime on more quality). That's worrying when security is involved, but it hasn't proven a moral or ethical problem for most industries absent of regulation (forced ethics, ha!), so... I think there's valid concern by OP.
As for passenger jets, the Airbus A320 (late 1980s) was the first commercial plane to have a "full" autopilot; all systems were electrical¹ (manoeuvering, thrust control, etc) which allowed the computer to integrate and manage it all. :)
It was tested a number of times by pilots for fun, from taking off to landing entirely on autopilot — ofc they're standing right there ready to take over if anything goes wrong but I've seen it first hand many times. We're talking commercial flights with passengers, it's 100% safe and actually quite "smooth" because the computer is so accurate.
Honestly, the problem is much, much, much easier for planes: a good GPS and it becomes quite the closed problem, and obviously 100% of autopiloted planes are simultaneously piloted by real humans... ready to take over. Yet a plane could technically land itself just fine if pilots were incapacitated, it really could. I suspect it did more than we know for many reasons. And when flying by instrument (means you see s__t), an autopilot is basically just a computer doing what a human would do slower by reading the same data (and maybe cross-checking with physical/manual instruments, but an autopilot doing the grunt work of stick-holding gives you more time to double, triple-check everything incidentally).
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[1]: Note that all systems are also doubled (even tripled) with mechanical (hydraulic etc) failovers, because obviously you can lose electricity in catastrophic situations, hence why it always seemed crazy to me that a planed requires software to fly properly instead of plain old good physics and mechanics).