It still boggles the mind that McConnell thought he could walk the tightrope and let Trump off the hook without it biting the GOP in the ass.
I'm sure McConnell isn't so naive as to think Trump would simply drift away, but I'm not sure it entirely dawned on him that Trump would try to control the 2024 primaries from the outset and make it seem like he's the only candidate. The GOP has to either find ways to keep Trump down or risk losing elections so long as Trump is capable of running... and even then, it might lose precisely because Trump could take his devotees with him.
The more time goes on, the more Trump comes across like Thanos — convinced he's 'inevitable,' but also a purely destructive force that will hurt the very things he thinks he's protecting.
I don't know if he thought Trump would drift away but I think he is correctly betting that Trump will not be the 2024 GOP nominee. Sure he might say that now but there are a number of good reasons he won't be:
1) He just lost as an incumbent, which is Jimmy Carter territory.
2) Right around this time in 2009 Sarah Palin was the presumptive GOP nominee - how did that turn out?
3) There's a pretty decent chance Trump and/or other members of his family will be under indictment in 2024.
4) I'm not sure if Trump has the appetite for another humiliation like 2020.
5) Given his poor health, morbid obesity, and poor diet there's a non-trivial chance he's dead by 2024.
While I could be wrong (thought he would lose in 2016!) I think there's a strong recency bias in who people think will be the nominee. Three years is a long time.