He was not useless. He was streakier. Like a lot of young players he was inconsistent and tended to show his best in the big games, when he was motivated and focused. With age and maturity his consistency level has improved.
Any top level striker could have done what Higuain did last season. Put Dzeko in Juve's team last year instead of Higuain and his numbers would've been equal or better. If Higuain left Juve tomorrow they wouldn't need Lewandowski or Suarez to replace him.
Higuain's a good, top level striker, but he's not irreplaceable and IMO not worth €90 million in and of himself. Now, there is a legitimate argument, it was worth that to Juve to weaken Napoli and reaffirm the Old Lady's status as Italy's big dog after selling Pogba. That's a slightly different argument. However, if Juve had bought Aubamayang last season and not Higuain they would've still done the double this season.