The demise of mainframes is entirely IBM's own doing. Mainframes are capable of incredible throughput, compute power, and reliability. They allow you to design applications with extremely tight SLA's without the complex and bloated distributed architectures you see spammed everywhere nowadays.

However... then you have to: 1) Deal with IBM's ridiculous sales staff. They're rarely technical and hype their random software offerings like there's no tomorrow 2) Straight up purchase these mainframes and put them in a datacenter somewhere. No easy way to just run one of these things on eg; AWS even if it's dedicated just to you. 3) IBM licensing is beyond stupid. The typical workloads people run today wouldn't be cost effective purely due to licensing. 4) The tooling is ancient and highly proprietary. People want to be able to write modern C++/Go/Rust/Java and if you want to use specialized libraries to interact with the hardware that's fine. But the current ecosystem is BAD. 5) How do you even learn to use Z/OS? If nobody can just play with it even in a free emulator then why would anyone even consider it?

If IBM bothered to address 1-5 maybe we'd actually see more people considering mainframes at larger companies. But they're digging their own grave.