The logic is pretty shortsighted, because you're looking at one particular snapshot in time, which happens to be one of franchise's recent low points.
Leading up to this moment, the Celtics had played in 3 of 4 conference finals series. They have two of the best young players in the league locked up for the next 3 and 5 years, respectively.
If Hayward had signed on for 4x$25 this offseason and Kemba looked like his 2019 self, who is making this argument? The Celtics haven't "gone for it" to this point because they've been in position to compete for the next decade. In other words, they haven't had to. All of a sudden you effectively remove two low-level all-stars from the equation and this is the year you get.