I don't necessarily disagree with your thought process. If we whiff on building a playoff competitive roster this offseason, and he decides not to accept a 4-year extension, it makes a lot of sense to sell high on Lauri.
I also don't assume we'll trade for an Ant, Luka or Giannis caliber guy. But Durant has been traded. Joel Embiid may be getting antsy. Devin Booker is almost certainly on the move. Jimmy Butler, Paul George, SGA, Damian Lillard, Anthony Davis, Kawhi Leonard, Donovan Mitchell, and Jrue Holiday have all been traded in recent years with multiple "prime" years left.
IMO, it all comes down to guaranteed years of team control we could get for a guy like that in a trade, but if Lauri re-signs, I'm personally willing to wait another ~1-2 years for the right cornerstone piece to come along. Pending OKC's moves, we'd be in the catbird seat given how many picks we could give up.
I don't hate the full-blown rebuild. It's fun to watch young guys develop and daydream about lotto balls bouncing your way for generational guys. But it can also be a recipe to suck for years, and years, and years.