"Porting" to upgraded hardware ... absolutely. I just never thought of it as "porting," in the commonly-accepted sense of the word, but really that's exactly what it is.
Ultimately everybody finds their niche and does the work they are most drawn to doing. Some like me are wretched outcasts...
Actually the demand for such people is considerable, although still niche - as long as you have an active $20k security clearance, which no company will pay for because without fluffy credentials (5+ years of "experience" in each of 916 languages and a b.s. [we all know what b.s. stands for]...
I have never seen nor heard of an actual app ever being ported. I'm sure it happens; I just don't see where. And I view the entire process as the ultimate in shoddy shortcuts and laziness. Just my personal opinion; we all have one.
By the logic of "portability first, quality second,"...
The impression I get (which may or may not be anywhere near correct) is that ARM processors have taken over the universe except for the PC and nothing else is used in any device except desktops and servers.
Hundreds of thousands of files? I'd say there's some room serious for improvement there. I've noticed that hours to compile one app is not uncommon in high level languages. For anything I create to require more than ten seconds to compile is exceedingly rare.
As for experience ... writing...
It's really not feasible to give a comprehensive answer to such a question. First, how large is the app? Second, what is it doing? Third, who is doing the coding?
Having used ASM exclusively for decades, I have a very comprehensive personal library of common functions that I use - something...
I write entire apps and drivers in all assembly (currently under Windows), and have done so for more than three decades. It continues today. Unless somebody else replying to this thread has done the same in the past, say, year, they are reciting their conditioning and they are not speaking...