Remember 2004, when Bush lost because everyone hated him and his foreign policy blunders? No way people would've re-elected an idiot like that.
Bush had a ~49% average approval rating on election day and got 51% of the vote. Obama 2012 had a ~50% average approval rating on election day and got 51% of the vote. If Trump has an approval rating of ~40% on election day, he's not really comparable to those two. We'll have a better idea in a year or two if Trump is ever going to recover, but it's worth pointing out that Trump has never once breached 50% approval across his entire presidency, and he's through a sixth of it by now.