This is an incredible team. They got a shot to win 70 games next year.
Are you talking about the Warriors or the Celtics? That is an exceptionally hot take for the C's, although I could see them winning 65.
To the OP's question, I don't think beating the Warriors would make this Celtics teams one of the best ever. However, it could mark the beginning of a Celtics dynasty. In the past up and coming teams overtaking the current champion has marked the start of a team's dominance.
I mean, if you grant the team good health and expected improvements from young guys like Brown and Tatum, is winning 70 games really a hot take? We mark all the boxes needed for a team to win that many games: 1) top-tier talent, 2) young All Stars in the making, 3) high-end role players and depth, 4) great coaching, 5) veteran leadership, 6) a two-way team, 7) size, length,and athleticism across the board,
excellent versatility, 9) a tight locker room, and 10) an easier conference schedule.
Given just decent health throughout the year, I really think 70 wins is a realistic goal.