No offense, but I'll believe it when I see it.
Jayson Tatum has played every game so far this year.
Jaylen Brown played 78 games last year and 67 games so far this year.
Al Horford has played 67 or more games in 9 out of his 11 seasons.
Before this season Hayward had never played fewer than 66 games in a season.
Kyrie has played 59 or more games 5/7 of his seasons in the NBA and was available for full playoff runs through the Finals last season and the season before. The season prior to that he was available in the playoffs through game 1 of the Finals.
I would say the biggest question mark out of those is Kyrie, but the only issues he's had with availability the past several years have stemmed from a single knee injury that happened in Game 1 of the 2015 Finals. By all accounts, this most recent procedure was something they expected all along to become necessary eventually. He elected to have it done now, probably, in light of the fact that the team was already missing Hayward, Smart, and Theis for most or all of a potential playoff run.
All in all, I don't see a good reason to be so skeptical that the team could get 60+ games from each of the five core guys mentioned above and have them available (i.e. able to play if not 100%) for the playoffs.