Then it will be teams trading away assets for less than a year of Giannis. Also a third of the league will be untradable then as free agents so a deal is actually much easier to construct now. At the draft some teams will want to see what happens in free agency. At free agency teams won't want to give up their drafted players.
There is always an excuse to wait but unless the Bucks can't get anywhere close to fair value I'd think they should get what they can now. Many teams would want to have Giannis for this year's playoffs and in the summer that value is gone.
With Giannis on an expiring I'm not sure teams will be much turned off by the prospect of Giannis leaving (nobody leaves in free agency any more) but the thought of having to pay him a massive new 5 year 300+ million contract might further give teams hesitation to fork over value.