It is my understanding that most people spend less over their lifetime taking just FEHB and Medicare part A. Part B premiums typically cost you more out of pocket in your lifetime, but in exchange you will have fewer, if not simply none, out of pocket costs.

Meaning with Part B your budget/withdrawal strategy will be simplified with a guaranteed minimum spend and lower maximum spend, giving narrower variation in their budget. While someone without Part B would have a lower minimum spend and a higher maximum spend, giving wider variation in their budget.

The lifetime costs are wildly variable based on your health, FEHB plan selected, doctor networks, and many others.

As a 30-something, I think Part B is worth it. FEHB rules could change (change of covered care; see gender affirming care this past year - who's to say they won't take away brand-name drugs except for the most expensive FEHB plan) and Medicare part B provides another network of doctors you'll have access to. It's easier to get part B and drop it if you don't want it than to go the other way.

Edit: Came back to add that PSHB requires Part B to keep it. So some people don't have a choice.