Fallon is evergreen, which helps. I'm not the biggest fan but I like him, yet NBC would be wise to keep the current POTUS as far away from the show as possible as it's already created backlash issues. Keeping him apolitical and still silly and likable will be the best way to ensure success for this attraction.
Ironically enough, when he did have Trump on before the election, he did get a ton of heat for being really light-hearted with the interview and playing with his hair instead of asking him serious questions.
He has the same problem as Leno - weak interviewer. Leno got hot in the early 90's because of the OJ trial (and Hugh Grant), seems like Colbert/Trump is swinging it more his way.
But yea, as the star of an attraction - he's perfect. Worse case scenario - if Fallon fails or gets fired - he can always be replaced quickly and they don't have to tie it to the Tonight Show.