I disagree. Acquiring Cousins for future picks and bit players, would absolutely give Boston a chance against Cleveland and Golden State. I mean this is a pretty solid rotation
Points - Thomas, Rozier
Wings - Bradley, Crowder, Brown, Smart
Bigs - Cousins, Horford, Olynyk
That 9 man rotation would cause both Cleveland and Golden State a lot of problems defensively and would absolutely crush a small ball death lineup. Draymond Green is a great defender, but he can't guard Cousins in the post. Even a bigger guy like Tristan Thompson can't handle Cousins in the post. The other four Boston starters all good enough shooters that they can't be left alone and thus, you would force GS and Cleveland to play big or get killed by Cousins, which takes away a lot of their offensive advantage.
I'm certainly not suggesting Boston would be favored against either Cleveland or Golden State (it would not be), but to suggest that Boston couldn't challenge them is just silly.