IMO, despite what some fans think the Suns have thought all of this through. It's not like they still don't have every other pick and options. Maybe the Suns think we have only a couple realistic windows left with Book?
We are in the first window and are trying to extend it with KD for the rest of this season and next (possibly a 3rd but I doubt it). I can see them, by the end of next year, making changes if they don't win a title. I can see Paul, Ayton and KD all being gone. Ayton possibly sooner if things completely implode next season. The Suns would then likely try and shed other salary as well to clear space and bring back assets.
Then they will have one last window with Book to retool. Books extension kicks in in the 2024-2025 season. I think I read he will take up 35% of our cap and be 27. That second window won't be very long because if the Suns ever consider moving Book for a boatload of assets it will have to be in his prime. The second window will likely be only 2 or 3 seasons tops. Beyond that Book isn't gong to stick around for another complete rebuild and will want to be moved anyway.
Unless we are winning titles, Book never finishes his extension contract as a Sun. The team will be ready for a complete rebuild by the end of the second window. I think the Suns have thought this all through with the time they have left with Book. You guys honestly think the Suns front office hasn't spent hours upon hours figuring all this out or at least contemplating?!!?