I agree with a few points the OP made, but "we already have championship caliber guards." Not quite, we have tiny guards, one is always injured, and can't develop his own reliable offensive game, and one specifically who is a major defensive liability, literally a target to take advantage of, and easily controlled in a playoff atmosphere. Re-watch the first two games of the Bulls series. Before Rondo got hurt.
I'll take the "bird in hand" every time. That bird is Fultz. Trading the number one pick for more future picks is crazy. We already have young picks stashed all over the world.
The Celtics as a team are NOT a "tick" away from being in the finals, we're far from being a team to contend with the Spurs, Cavs or Warriors. We need a step-up in positional size, and talent at a few positions.