I agree, the lack of depth forced the top 6 players in the rotation to play way too many minutes, and it caught up with them.
The Heat made moves mid-season to add Crowder and Iguodala. Good chance they don't make it to the Finals if they didn't make that trade.
The Celtics in this playoff run had:
Tatum - All-NBA two way wing
(25.7 pts, 10.0 reb, 5.7 ast, 7.2 FTA, 56.3% TS)
Brown - All-Star caliber two way wing
(21.8 pts, 7.5 reb, 2.3 ast, 3.7 FTA, 58.6% TS)
Kemba - Average / above-average starting guard
(19.6 pts, 5.1 ast, 4.1 reb, 4.8 FTA, 57.3% TS)
Smart - Elite defensive role player
(14.5 pts, 5.2 reb, 4.6 ast, 3.3 FTA, 55.2% TS)
Apart from that you have
Theis - Backup center who is out of his depth as a starter
(8.9 pts, 7.1 reb, 52.1% FG in 28 mpg)
Wanamaker - limited game manager backup point
(4.9 pts, 2.0 reb, 1.8 ast on 48 / 44 / 88 shooting in 16 mpg)
Then you have
Rob / Grant / Semi - Limited young role players averaging ~10 mpg
Kanter - Matchup specific third string center (9 mpg in 11 games)
Hayward played in 5 games and overall was pretty awful (10.8 pts 4.0 reb 2.8 ast on 40 / 29 / 88 shooting in 31 mpg)
Like Roy said ... you just need more guys you can rely on to play the role they're being asked to play in order to have a shot at a title.
It is really amazing they got this close.
The 2018 team got a bit closer to the Finals and was sort of comparable in terms of lacking depth. But that team at least had 7 guys playing 20+ minutes a night in roles that made sense.
Biggest difference I see with that team is that Horford was the unquestioned leader of the group on both ends of the floor. He had years and years of playoff experience even though he'd never made it beyond the ECF.