To my eyes, we still develop software in plain text and the same languages are still dominant. It's still Linux and http.
It's not just Linux and http anymore. It's frameworks towering up to the heavens.
On the one hand I agree with you, in that these liabilities you describe are much like the legacy systems I see in our local banking sector.
On the other hand, these legacy systems just work. And adopting newer / more modern systems might definitely have advantages, but doing so creates a bunch of other liabilities - as I've seen take place in our new startup banks.
Stuck between a rock and a hard place.
Of course that's just in banking. I most certainly don't have the experience to comment on whether the same liability tradeoff would happen at FAANGs.
Its not being stuck between a rock and a hard place, you just need strong management that understands that changing whole systems is never a good idea without clear benefit, other than "technical debt" which is such a bad term for reliable software. Software doesn't age, im not sure how software engineers don't understand this, or maybe they do and want to write more s/w.
And that's besides tribal knowledge that goes away as people leave or retire.