I agree that you don't want to spend time becoming super familiar with everything that you use but you should ALWAYS have a high level idea of what's happening under the hood. When you don't it inevitable leads to garbage in garbage out.
Way before the arrival of personal computers, it was clear for me to realize that I would be highly disappointed if average programmer abilities declined half as far as they have by now.
Some of the most coherent technical projects are the result of a single individual's outstanding vision.
Occasionally, the project can be brought to completion by that one individual, in ways that simply could not be exceeded alternatively, all the way from from fundamental algorithms through coding to UX and design.
Additional engineers could be considered useful or essential simply to accelerate the completion or launch.
When the second engineer is brought on board, it may be equally performant to have them come fully up to speed on everything the first engineer is doing, compared to having them concentrate on disparate but complementary needs which need to be brought to the table in addition.
If they both remain fully versed, then either one can do anything that comes up. Sometimes they can work on the same thing together when needed, other times doing completely different things to make progress in two areas simultaneously.
You're going to end up with the same product either way, the second engineer just allows an earlier launch date. But truly never within half the calendar time, those man-months are legendarily mythical.
For projects beyond a certain size more engineers are simply essential.
Then ask yourself what kind of team would you rather have?
All engineers who could contribute to any part of the whole enchilada from primordial logic through unexceeded UI/UX?
Or at the opposite end of the spectrum, all engineers who are so specialized that the majority of them have no involvement whatsoever with things like UI/UX?
Assuming you're delivering as satisfactory a product as can be done by the same number of engineers in the same time frame.
You're NOT going to end up with the same product either way.
Now aren't you glad it's a spectrum so it gives you an infinitely greater number of choices other than the optimum?
As long as the number of excellent programmers is steady or increasing, I'm not too disappointed if there's a bunch of average ones too, as long as those average ones have great tools that let them still make good software.
It does seem like some of the very top innovative projects are done by one person.
I have very little direct experience with software small enough for an individual to understand, but it seems like a lot of our modern mega-apps are elaborations on one really great coder's innovation from the 80s, and real knock your socks off innovation only happens every few years.
Engineers that don't understand or care about UX can be a really bad problem, attempting to bolt on a UI on something meant to be a command line suite is usually highly leaky.
The opposite seems to be slightly less of a problem, to a point, almost nobody writes sorting algorithms, and writing your own database is usually just needless incompatibility.
I definitely am glad it's a spectrum, because having absolutely zero idea about the real context gets you in trouble, and stuff like media codecs are hard enough we'd lose half the worlds devs(including me) if you needed to understand them to use them.