I just don't think you can pay at absolute best the 4th best player on a contender 20 million a year (and he is probably more suited as the 5th best player on a contender), especially when you aren't a contender. That is how you end up perpetually in mediocrity.
And it is easy to say you can move that contract now, but there is absolutely no guarantee you can move that contract. 40 million with 2 years left is still a lot of money for a role player. The teams that would take that contract (without a 1st or something like that) often don't have the pieces available to move to do so.
Fournier is not going to raise the ceiling of Boston this year and he could harm the team's ability to really improve next summer, so there was absolutely no reason to sign him.
That's only true if it's not a movable contract or you're using cap space to get said player. Otherwise it's merely a matter of ownership's willingness to spend for the most part.
Boston is using cap space to get him though. Not this year, but next year. And as I said, I wouldn't just bet that the contract is movable, at least without including other assets.
Probably assets you'd need to get rid off anyways if you're using cap space next year. Next year the contract would basically be a 2-year contract, that should be easy to move with minimum effort... $20 million per year is not an absurd amount to move for a player that can contribute in this day and age.
It also opens doors for S&T possibilities or in season trades this year as well, which is just as important.
So in the Celtics situation, I think it would've been worth keeping. Not only to be more competitive this year, but because the opportunity cost is low when you consider trade scenarios vs. cap space. It's not the definitive course, but a very valid avenue, one which I think would be better than not keeping him. Which in all right now is a moot point.
As for the Knicks I like it less, but I think it's still a good contract in the end regardless, but the Knicks don't have much of a core of players to use space this way in my opinion. Now that they added Kemba et al. it may payoff, and be a worthwhile risk for them. But Knicks in general are a good free-agent destination, so the risk is a bit higher with them, their problem has always been management.
The only real reason using cap space for the Celtics right now is that there seems to be a wink wink understanding that Beal wants to come here, otherwise playing the cap space game for the Celtics is a very risky endeavor, the better bet would've been playing above the cap with as many assets as possible, particularly with players who can actually play and not solely development options.