Talking about a single mainframe lasting 20+ years is disingenuous given mainframes are not normal servers and inherently have a longer lifespan. Even then 20+ years is an exceptionally long operation for one of these, not because of hardware limitations, but because it normally does not make financial sense. Mainframes typically run legacy systems so they are the one place these kinds of financial rules don't apply.
The average operational lifespan of a server is still 5-6 years as it's always been. Some businesses are replaced every 3 years. A quick Google search would tell you that. That's not a limit inherent to the hardware, but simply how long they are warranted and deployed for in most instances. As I explained before it doesn't make sense to keep servers around for that long when more modern options are available. Saying they "would still work perfectly fine" doesn't really mean anything outside of the hobbyist and used server market.
LLMs haven't reached a pause in performance and they are only a category of AI models. If you actually kept track of advancements instead of sitting here whining about it you would have seen more has been happening in even just the last year then just throwing data and compute at the problem. I find your intentional ignorance to be morally insulting.