"perpetual rebuild"
Let's look are quick rebuilds. Here are the teams in the last 10 years who won 25 or fewer games in one season and two years later won 48 or more and the primary reason for the quick rebound:
Celtics (06-08): KG, Ray Allen trade
OKC (07-09): Durant, Westbrook (both top 5 picks)
Toronto (11-13): Two top 11 picks
Golden State (11-13): 4 top 11 picks
Cleveland (12-14): LeBron, Irving, #1 for Love (two #1 overall picks)
Boston (13-15): One top 11 pick
Notice anything? The Cs resurgence (it ain't a rebuild any more) is quite remarkable. First of all it was exceptionally fast. That's 6 teams in 10 years. So this isn't something that happens often. Furthermore, the way it happened is almost unique. Theirs and Toronto's are the only rebuilds where the teams didn't get multiple cracks at top 10 picks. So that's two teams (and three times) in the last decade who have been able to spin it around quickly without building through the draft.
The team is on pace for 52 wins, #2 or #3 seed in the Conference and still has very likely two more top 5 picks to go along with Brown (who is already proving you wrong). Perpetual rebuild? That couldn't be further from the truth. It's an ever-increasing refinement of an already very good team with the assets that "rebuilding" teams only wish they had.