People really don't appreciate this reality enough. Unwinding 1-2 years of unenforced border would be messy. Every additional year makes it messier. Undoing multiple generations of border failures was never going to be pretty.
This is like someone who spends $100 more than he earns every month, and takes out a loan to cover the difference. He does this for 40 years straight. At the end someone forces him to pay down his $100k+ debt, and he's shocked that the minimum payment is more than his rent. "Why is this so extreme? I only overspent a little, I could afford to!"
Trump has 4 years to address 6 decades of damage, before a Democrat might come into power and shut it all down. He knows he has to hit hard to undo any of it, to any appreciable degree. If the Democrats or Republicans could be trusted to carry out slower, thoughtful, longer term reform... but they've been feckless double talking weasels since Reagan. Nobody believes them any more, and nobody should trust them with this task.