Also, a largely Westbrook-less Thunder won 59 games, so I don't quite agree with the "the Celtics are better than that team" argument.
Without Westbrook and Durant, the Thunder were awful. The Celtics won 48 games and don't need to gut their roster to add Durant. They can even add another max while keeping their corps together.
Yes, the Thunder without two of their best players were awful. However, you specifically were referring to the Westbrook-less Thunder, i.e. the Thunder with Durant, but no Westbrook. They finished that year with a considerably better record than the Celtics did.
And it's not exactly surprising that a team without its two best players would be bad. You think the Warriors would fare well without Steph and Draymond? Or the Cavs without LeBron and Kyrie? The Clippers without CP3 and Blake? Yeah, the Thunder were bad without their two best players, as would be virtually every other good team in the league.