Can we agree that the objective with this pick (perhaps barring trade) is to get the single best player that yields the singular best results and that this is the entire purpose of having the top pick, as opposed to just getting a good player?
Sure, in a vacuum that's correct. Big Picture, however, that's not necessarily true.
The general objective is to get the best team possible for as long as possible, with "best" being defined as giving you the best shot at a championship. If people believe that three high picks is enough to achieve such a goal, they're rather naive, I'm afraid. The Wizards, for example, had three relatively high draft picks, and hit on all three of them in Wall, Beal and Porter. Do they even remotely look like a championship contender to you?
Moreover, if we want a real contender for years to come, conventional wisdom suggests that you need at least one "transcendental" talent on your roster. At least one of the very best players in the league. So the question becomes: Even if Fultz reaches his potential, will he be one of those rare beasts? Apparently, the Celtics don't believe he will (nor do most NBA scouts), or at least they don't think it's very likely.
So, our intermediate objective is to figure out how we can get one of those super-duper-stars.
Trading for multiple future picks isn't about watering down our assets, or trading a Dollar for 2 dimes, it's about acquiring the assets necessary to potentially trade for such a player if/when he becomes available.
The NBA trade system is usually a pre-currency market with fixed commodities. Basically, you trade chickens for milk, milk for eggs, and eggs for bread, depending on what you need at the moment, just like back in the day. Draft Picks are the closest thing the NBA has to a currency. As soon as the pick turns into a player, his value is not what people project into it (like we all do right now), instead it's a much more narrowly defined, a specific value. Do you rather have the 2013 #1 pick, or Anthony Bennett?
What that means is that if we pick Fultz, we better are committed to him, and even if we consider the possibility of including him in a trade package down the road for, say, Anthony Davis, we have to hope that the Pelicans even want him.
So, considering our main objective (winning championships, duh), the Juggernaut in Oakland, and if we conclude that Fultz isn't on the Lebron/Durant/Davis level, then why commit to him and not kick the can down the road and retain flexibility for when such a player becomes available, all while squeezing as much value as we can out of the
projected value of the #1 pick, instead of the "fixed" value the pick has once we make it?