Disclaimer: I know very little about land weapon systems, but I will try to explain what the situation is like with aircraft, because you mentioned probably the best example for it: the B-52.

The reason is that weapons developed immediately after World War 2 utilized—in theory, at least—the complete set of lessons of WW2, which was not the case with weapons designed before the war’s conclusion (well, duh). And since WW2 introduced a number of significant technological advancements, it was necessary to have weaponry that could either take advantage of or counter said advancements.

At its very core your question can be boiled down to: what is the nature of an arms race?

Take bomber aircraft, for example. The B-29 Superfortress, which was by far the most potent bomber of the war even without nuclear bombs, was introduced in 1944 and was kept in combat service throughout the Korean War, but at this point it had become abundantly clear that in a hypothetical WW3 scenario the US Air Force would need to conduct mass nuclear strikes against the USSR (and potentially China), a task for which the B-29 was not suited in the least. That is why the United States was developing newer, even more potent bombers: the Convair B-36, the Boeing B-47, and eventually the Boeing B-52 and Convair B-58.

But while the B-58 remained in service for about a decade, the B-52 is still being upgraded. A major program is underway as I write this, which will probably allow the Stratofortress to remain in service a century after its maiden flight (we’re 32 years, less a month, from that point). So why did the B-52 succeed in achieving longevity where the -36, -47, and -58 failed? Why did the USAF not phase it out?

Certainly not for lack of trying. But while all of its potential successors have always been problematic in one way or another, the B-52 retained its most important feature: it was always at least good enough for what was required of it. You needed to send waves of bombers armed with nukes to Moscow? The B-52 could do it. You needed to bomb the North Vietnamese with conventional bombs? The B-52 could do it. You needed to drop two or three bombs on a Taliban holdout? The B-52 could do it. So while there was desire to substitute it with something more modern, there was no need to substitute it, and resources could be better utilized elsewhere.

All this being said, in the context of the B-52 we need to look at one crucial factor: the B-52Hs of today have little in common with the B-52As used for flight testing in the early 1950s. While the airframe as such was eminently useful, its insides were upgraded over and over again: into the B-52B, C, D, E, F, G, and eventually H. And as the re-engining program continues, we’ll most likely get to see the B-52J. Each variant of the Stratofortress received better avionics, integration with more modern weapons, better engines, internal structural upgrades and so on. The strength of the B-52 lay in the fact that all of these upgrades were possible without prohibitive costs.

Therefore, I would suggest that the question one needs to answer in order to determine how long a piece of equipment will remain in service is: how possible will it be to upgrade this piece of equipment to retain its relevancy in a modern battlefield.

There are factors which cause the answer to be: “not possible at all.”

No God-ordained reason exists for why the Me 262 or Gloster Meteor couldn’t be flying even today and be successful as a combat aircraft. But what upgrades would be needed in order for that to happen? Well, in order to facilitate supersonic flight, first of all the airframe would have to be redesigned so as to conform to the area rule, which had not been properly discovered until the late 1940s. This already means that we need to design a brand new aircraft (this is exactly the basis of how the F-102 Delta Dagger turned into the F-106 Delta Dart). But then we also need better, more reliable engines. We need to stuff a radar, preferably a modern AESA radar, in the nose which has no room for said radar. We also need room for larger fuel tanks. And so on, and so forth. The amount of changes required would quite literally result in an aircraft that had nothing in common with its supposed previous version. So you just design something new, because its easier, cheaper, and more efficient due to the fact that all the new technology will be integrated into the airframe from the outset.

Still, you need to consider one more important factor: the B-52 is a freak. It’s one of just a handful of such old combat aircraft surviving in service in its own country. Off the top of my head, it’s in a group of two: with the Tu-95, which had since been transformed from a pure bomber into solely a “strategic missile carrier.” The MiG-21, which is from roughly the same era, is still alive and kicking, but only in smaller or poorer, or less technologically developed countries, or those which have less pressing military needs. Again: for those countries the MiG-21 with a set of upgrades is still good enough. Or even when it is not, it’s the best they can afford at the moment. But the fact that the MiG-21 survived until now when the MiG-15 did not is that the -21 allowed for upgrades which preserved its status as a weapon, not just a target for slightly more modern fighters.