People need to stop saying that. We should all hope to the very core of our beings that Trump lives out his term for exactly one reason: 2028.
A mentally declining Donald Trump will be little more than his malignant narcissism. People may explain to him why he can't have a third term and that he has to smooth the transition to a new Republican standard-bearer, . . . but this is Donald Trump we're talking about. The notion that the Republican Party--much less the United States--could move on without him is more than he can possibly bear. There is simply no way he can gracefully hand over the Party and his support to someone else.
So starting in 2027, and in ways small and large, he will undermine and diminish whoever is leading the Repubican primary--essentially trying to position himself as the only only possible nominee. If it comes down to a contest between Cruz and Vance, for instance, he will switch his supposed support at least a dozen times. Someone will come into the Convention with enough delegates to secure the nomination, but it probably won't happen until late. And even then, it will be after a bitter primary contest.
And even after someone gets the nomination, Trump will subtly (and not so subtly) undermine the nominee. Until somewhere around October 10 he'll drop the idea that people ought to write him in. And a bunch of his idiot supporters will. It won't be a huge number. But it will be enough to make a material difference in anything approaching a swing state.
And ultimately, that's what Trump wants. He wants to be the last Republican president. Because the notion that the party can exist without him is noxious to his ego.