I get the general feeling that Boston isnt going to resign IT4 after next season anyway. Fultz steps in as the starting PG for him.
I have no idea where your feeling is coming from, other than a place of misunderstanding.
Currently, the top 10 2017 talent is at point guard, with a few interesting small forwards in the mix. Giles has shown no signs he's still the elite talent he was pre-injury. The wing players are talented, but no one looks like they have substantially more potential than Jaylen Brown. Do we really want to pick another raw young wing when we have a solid starter and a promising #3 draft choice on the bench?
On the other hand, their are a few point guards that have quite a bit more potential to be two-way superstars, with Fultz leading the pack. He would pair with Bradley perfectly because he's big and long and probably is better suited to covering shooting guards, but should play point on offense. The same is true of Ball, Fox and Ntilikina.
Unless a big man emerges that isn't evident now, we are most likely to select a point guard. So it comes down to what Ainge will be offered for his assets, and that includes both the choice and IT.
We need a home run at some point, and we aren't going to win a title with Isaiah as our best player. The issues with paying him big money well into his 30's have been thoroughly discussed.
The only guy I see us having any chance at trading for that might make us a top tier contender is Cousins, and getting him without gutting the team is a long shot. A player like Jimmy Butler or Gordon Hayward won't get us past Cleveland
Unless we get lucky, the smart play is to build a monster that emerges in 3-4 years, after LeBron returns to earth.
A scenario where Ainge selects Fultz or Ball and trades a package that includes IT is as likely as any.