This is just one person's opinion. I think that, in terms of relevance to computing in general, IBM peaked in the 1990s with the Power processor and its various follow-ons. That architecture really was (and is) very good: it's clean; it's uncluttered; it delivered high performance computing; it was somewhat daring in its SMP memory system (again, enabling high performance but requiring thoughtful system programming.)
It was intriguing enough to get Apple to make it their Mac processor for a while. One variant, CELL, was used by SONY in some generations of Play Station. There are also specialized embedded versions. Even now, if you go to top500.org, this architecture is in the 9th most powerful supercomputer as of June 2024. "Back in the day," let's say 2006, this architecture was used in the first, second and third most powerful supercomputer.
Also, IBM recognized that the only operating system that mattered in this space was Unix and they bought and maintained a standard Unix system and when Linux came along, they were happy to adopt that too. (They also have a Linux on Z that no one uses.) In the original architecture, "big-endian" was slightly more preferable but, over time, the processor became completely agnostic.
I think that those were the best days for IBM. So what happened? Let's start with the cell processor. That was an amazing machine and very difficult to program. In a very real way, it foreshadowed the world we live in today with a CPU and a GPU mining bitcoins and building an LLM AI monster. But it was not easy to program. Next, let's look at what Intel and later ARM did. They attacked in a couple of directions. First, the price to performance ratio was always IBM's Achilles heel. When you have superior performance, you can ignore this problem. But somewhere between the 386 and today, the performance of these other parts became more than close enough. (I'll also mention that, as a computing device, Z cannot hold a candle to any of these. It has its strengths but raw computing power is not one.) A second major problem is the amount of (electric) power the Power family consumes. Power consumption (and the heat generated from that consumption) mean that you cannot have a battery operated version. (Which is probably why Apple moved on.)
IBM does have other, more current contributions to computer science. They claim to be both AI and quantum computing leaders. But, for my money, the best thing the ever really did (in terms of computing) was the Power family.