If we sign Thomas to a max deal, especially the 5 year, we are in danger of losing that flexibility. A long term deal for a player who will be 29.5 years old and who is tremendously short is a contract that will lose value. A 4-year deal for a player who will be 27.5 years old and whose game isn't overly reliant on athleticism and speed is a contract that has an acceptable probability to keep its value.
Explain this. Explain in detail just how signing Thomas to a max deal interferes in any way with possible roster moves.
Signing IT to a max next summer has zero bearing on who we can sign this summer (unless you advocate trading IT's salary now in a dump to get it off the books for cap room).
By next summer, whether we sign a max free agent now (i.e., Hayward/Millsap/other) or instead just re-sign our own free agents (i.e., Olynyk, Jerebko) we will be over the cap either way.
At that point, whether we sign Isaiah to a max contract or to another rookie-level deal has ZERO impact on whether we can or will re-sign any of our other free agents (AB, MS) or new rookies (BKN 18, BOS 18). We won't be able to sign any external FA because we will be over the cap anyway.
The only trade-off in consideration regarding Isaiah's, Avery's & Marcus' contracts will be how much Wyc is willing to pay luxury tax.
Otherwise, no CBA roster rule prevents signing all three of them. (Though possibly one might have been traded this summer already in order to create cap room now. That just relaxes the tax constraint but doesn't really change things much.)
Horford's deal would still be in effect for the first two years of an IT contract so Boston would be carrying three max deals (Horford, Isaiah and presumably Hayward) for two years before they would drop back below the luxury tax threshold (though they would still likely be above the salary cap). They would at that point need to start re-signing guys like Jaylen Brown to his first post-rookie deal. That may or may not push us back up above the luxury tax, but nothing will prevent us from doing so other than a desire to save luxury tax.
The following year, 'Hayward's contract would come off the books. At that point he may or may not be re-signed to a new contract. It would likely still be a large contract, but probably a smaller percentage of cap than a max deal. Fultz, Zizic and possibly Yabusele might also need to be re-signed at that point.
Where in this are we restricted from doing something that we would be able to do if we DON'T sign Thomas to a 'max' contract?
What would be different if we signed him to say, a 12M per year contract? Or if we let him WALK and only decide to pay Avery & Marcus. I'll tell you the answer:
Only one major thing changes -- how much luxury tax is paid. (Oh, and loss of IT if we let him walk, of course)
Now, if your overriding concern is to save Wyc's pocketbook, then that should be important to you.
If you believe we should pay fair market value for our players and that our focus should be on constructing the roster based on basketball reasons, then it should not.