Yes he could be beaten. Incredible ring general he was, yes he was beatable, all fighters are from the best to the worst. Incredible boxer, outstanding skill, but yes he could have been beaten.

Holyfield I think is a guy in ANY era of boxing he'd give the best of the best of the best a hard time. Joe Louis was like that, Marciano to an extent (given he wasn't even really heavyweight sized) was like that. Not that they'd be the ones to beat Ali, but they were just not easy fights at any point in their careers. Foreman kind of evolved and his raw ability as a young fighter is only rivaled by Wilder.