Boston isn't winning the title this year. Never had a real shot with the Big 3 the team has. So why hamstring the team by adding big money role players that aren't going to do anything.
About when should we expect push for a title?
When Tatum starts approaching top 5 status, which may be next year but probably more like the 22-23 season (if he elevates that high at all). You need that 1st Team All NBA guy to really have a shot at being a multi-year true contender and Brown needs to get up into that 3rd Team All NBA range (or higher) for Boston to really feel comfortable about contending. I mean the best player in the world doesn't have a top 15 teammate and he hasn't even made the Finals yet (and if you want to call Giannis top 5, and not the best, that is fine - the point still stands). I mean look at the teams that have won titles. They all basically have the 1st Team All NBA guy surrounded by at least 1, and often 2, other All NBA guys (or at least the talent of All NBA guys even if they didn't make it that particular year). That is reality. Top tier talent wins, and Boston's young guys aren't at that level yet so Boston isn't going to a contender until the young guys hit that level.
Middleton was absolutely an All-NBA calibre player last year no matter how much you want to crap on Giannis' supporting cast, just because the Bucks have lost to two really strong teams in the playoffs (the Heat definitely had an unexpected run, but the totality of what they had really looks like something akin to the '79 Sonics with Butler as their All-NBA guy and two All-Star level players in Bam and Dragic with a strong collection of role players) doesn't make those Bucks squads non-contenders.
The Bucks are contenders because Giannis is that good, but the fact they haven't made the Finals yet, absolutely furthers the point I was making. Butler is a top 10 player and Bam is top 15 or so. Those two are both better than Middleton, though worse than Giannis. Dragic was the 5th best player in that series. The year before they lost to the Raptors with a player equal or near equal to Giannis, but with Lowry and Siakam both better or in the range of Middleton.
The problem with the Bucks is Middleton hasn't been a good enough 2nd when the 3rd player on the team was a flawed player like Bledsoe. I like the Bucks a lot more this year with Holiday. The Middleton and Holiday duo as the 2nd and 3rd best player could be good enough to get the Bucks to the Finals, though as we saw last night they aren't going to beat a team like the Lakers and with the Nets being potentially really good, the Sixers looking better, and the Heat still being around it won't be an easy path for them. If the Bucks had a real and legit All NBA player as the 2nd option, they'd quite simply be in much better shape.