Shooting and rebounding aren't revolutionary ideas in roster construction, though. The types of players you are describing here are guys who would fit in any system, with any PG. There are other skills that become less essential on a team with one of the most efficient volume scorers in the league.
And you ignore completely that this Celtic team is, in many ways, constructed exactly opposite to what would be best for a team with IT. They are arguably built to exacerbate his limitations. They aren't full of shooters, they don't have even one above average rebounding/shotblocking big, etc.
For example, how many teams give significant run to a 6'3" non-shooter at SF, other than the Celtics? Is that ultimately the role Marcus Smart would fill if he moves on (or if the Celtics get rid of IT)?
Conversely, what examples are there of successful teams with just one star player? Paul Pierce, to name one obvious HoF worthy one, didn't have a great track record of team success as the only guy. I'd trade IT happily for a mid-career version of Pierce, but 1) that, or anything comparable, ain't happening; 2) even as much of an upgrade as that doesn't put this team in position to challenge Cleveland/Golden State.
IT's limitations are visible to all and exacerbated by the current Celtics roster. Trading him for fair value in a way that doesn't cripple the Celtics offense is going to be hard to do. I'd rather at least take a stab at adding talent to him, via draft or free agency, using the resources the Celtics do have at their disposal, than take a deliberate step backward.