Yeah, both Hayward and Monroe are well below all star level. I don't think Monroe is even one of the 20 best big men in the NBA. Among the 4s and 5s, i would slot him behind Drummond, Randolph, Millsap, Ibaka, Pau Gasol, etc.
Monroe is maybe the 45-50th best player over all in the NBA. And Hayward is even below that. I would rank Hayward around guys like Batum, David Lee, Oladipo, Conley, etc. nice players, but well down the ladder when it comes to cracking an all star roster.
I can see being reserved about building a team around a bunch of sub all stars. The Utah Jazz appear locked on the suball star team path. Favors, Kanter, and Hayward are nice players, but none of them are game changeling difference makers.
That could place a hard ceiling on just how good Boston (or Utah) can get.
The Celtics would still have their stack of Brooklyn picks though. Monroe plus picks could yield a Bosh, Noah, Marc Gasol, etc in a year or 2. Those players are legit all stars and/or all defensive players.