Of course Celtics shouldn't be interested in that offer as you are right about if in case Durant decides to go again soon, Celtics will have almost nothing and they would destroy an already successful team which just got to the NBA Finals.
I also see now a news report that for an offer for Durant, Nets asked Jaylen Brown, Marcus Smart and bunch of draft rights. I think this is getting ridiculous. I know how Durant is an important player but come on. Celtics were in the NBA Finals. I don't even understand why Celtics even bothered by these trade talks.
It is not surprising that the Nets are asking so much for Durant because they don't want to end up with nothing. At the same time, it would be expected if the Celtics' management doesn't accept any of the Nets' offers. Honestly, I'm leaning towards the Celtics don't need Durant at all because they already have good team chemistry, and signing Durant can only make it worse.