It's simple, Thomas and Bradley can become free agents, if htey draft Fultz, they lose one of the others in a year. And they have Smart (#6) as well.
So Fultz's value to Boston wasn't that of a #1 pick because it would be shuffling pieces, not upgrading at a position of need.
BC played it cool, didn't panic and offer too much, and made sure he protected #1 in 2018.
In two years, Sacramento will have #5 and #10 this year, their first next year, so odds are while they'll still be a lottery pick team, they won't be terrible.
With the big three in place, plus Saric, RoCo and TLC, Sixers can now focus on building up the bench with 3 1st rd picks the next two years, 4 2nd rd picks this year, and 6 more the next two years.
That allows you to add D-league prospects, accumulate Euro stashs, trade for short-term patches and trade forward in the draft.