IMO what made the show work originally was its "anyone can play" aspect, thanks in part to the phone game. You didn't get so many out-of-work actors and caricatures. Maybe it was a product of its time, but IMO the overuse of the celebrity format killed the charm. The first celebrity tournament with Ray Romano, Norm MacDonald, etc. was fun, but it became a crutch.
Other than ratings, I dunno what killed the daytime show other than it got stale and turned into a generic trivia game. That and the fact that a million dollars isn't really a novelty prize for a game show anymore. It would be great to win, but so many shows have offered it as a top prize. IMO, winning seven-figures has become so routine on other shows that even if someone had won the top prize it was no longer a watercooler moment.
Terry was an awful fit for the show because he yelled too much. I don't remember much about Cedric, but it felt like the producers only wanted him because Steve Harvey was doing so well on Feud, which is apples and oranges given the two formats.
As for Shaq, he's great on Inside the NBA, but whether he would make for a good question reader remains to be seen. Obviously, time will tell but it's not like this is the first or the last time someone felt uneasy about a hosting decision.