Expanding on my short comment earlier: a turboprop B-52 could do everything that the jet B-52 does
today. It could not do everything that the jet B-52 did in the 1960s and especially 1970s as a penetrating bomber in the nuclear role, and USAF doctrine will not tolerate that.
The Soviets got away with the Tu-95 for so long because they didn't go in for penetrating strategic bombers. Instead, the Tu-95 was a standoff missile carrier for virtually all of its' service, as a supplement to ballistic missiles.
Since the USAF will not go down that route, the turboprop B-52 will need replacing before its' vulnerability issues become crippling. I'd expect the replacement to look a lot like a B-1A, but enter service in the late 1960s; since AMSA was widely considered to stand for America's Most Studied Aircraft, that timescale isn't challenging in the slightest.
It's also highly likely that the B-58 is procured in greater numbers; you might even see something similar to the B-36/B-47 pairing, with something like 300 long-range B-52s supplementing a short-range force of about 1,200 B-58s.
There would also be major knock-on consequences for air travel - no B-52 may mean no KC-135, undercutting the Boeing 707. It'll still exist, but it won't have the advantage of the big tanker order.
The C-130 has been used a bomb-truck into the 90's, so why not a turbo-prop B-52?
Off on a tangent (it's what I do..), with the extraordinary cost of developing bombers to be used against topline militaries; could you convert any of the modern transport aircraft to fill the bomb-truck/cruise missile carrier for use in secondary battlegrounds?
Yes - Boeing has proposed doing just that with the C-17, and Lockheed proposed doing it with the C-141. In the latter case, it seems to have been an alternative to the B-52D BIG BELLY for Vietnam.
Trouble is, it's a lot of cost to go to for an aircraft that can't contribute to a war against a near-peer competitor.