I think it's supposed to like the en- in entrenched. To be trenched would be to be put in a trench but to be entrenched, well, that's when you already got trenched. Now have been inside the trench. The tench is all around you and you're stuck in a mire. So things get shittified, then they stay that way until they just get mired, I mean just completely covered, in shit. At that point they are enshittified. Trenched is a made up word, but so is enshittified. I guess Doctorow could have called it shitslapped or shatblasted or whatever.