As long as Giannis remains in the conversation for best player in the league, I think it's doable, but it's obviously going to get a lot more difficult as Middleton, Brook, and Lillard age out and it wouldn't be surprising if there comes a time where Giannis is in a similar position to what Lillard had in Portland, where he had to decide between playing on a real contender outside of his home team, or staying home and playing for a team that's a notch or two below contender status. I think getting to that point in a few years is probably the most likely scenario, but every time it's felt like Milwaukee may have hit their ceiling, Horst has found a way to upgrade the team in a significant way.
Would really be nice if Beauchamp or Andre Jackson turn out to be starting-caliber players lol, either to help replace some of the older guys or to be used as trade chips.
If there comes a day where Giannis does end up leaving, he's already given Milwaukee more than enough; I won't fault him at all. Definitely hope he's the rare player that stays with one team though.