Bucks have no path to pursue upgrades to the roster as they lack picks and cap space. With Dame's recent achilles tear and him being under contract for 2 years, trading Giannis is the best way to rebuild.
This is the minimum they should get back for Giannis, which is getting control of all their own 1st round picks. They should get back more 2nd round picks and 1st round picks, but this is the minimum.