They might win a few titles, but I don't think they're set up forever (I know that's hyperbole). The same reason why the Warriors are now just a mediocre team, will happen to the Thunder. Payroll and lux tax are real shit.
as far as age, the Warriors won their last title when Curry was 34 while Klay and Draymond were 32
for OKC....SGA is 27, Jalen Williams 24, Holmgren 23, Dort 26, Wallace 22, Mitchell 23, Hartenstein 27. So, going by ages at least, the window for an OKC dynasty seems 7-9 years long, not counting last season
but you're right about tax and aprons. Somehow, right now, OKC is 1M below the tax line with a 187M payroll. But next season, when the 46M/year each extensions for Holmgren and Williams kick in, OKC will be 23M ABOVE the 2nd apron with a payroll of 246M. The estimated tax bill alone will be 181M. Patroll + Tax will be around 427M. I don't know how wealthy the OKC owner is but I'd imagine he views that as unsustainable
it gets slightly better in the 2027-28 season but only because Hartenstein, Dort, and Kenrich Williams are UFA. The offset is that is the year SGA's 4-year extension starts at about 70M/year, So yeah, OKC is facing losing critical players but still be floating around the 2nd apron, or higher
the one way that OKC can add talent and actually cut payroll is the draft, and Presti's front office has been masterful at collecting picks an evaluating talent:
they have no outgoing picks by themselves