IBM mainframes were for a very long time built and sold on the premise of providing reliable emulation of previous generations weren't they.

Still are.

The latest IBM z/Architecture mainframe (released in 2024) will natively run binaries compiled for the original IBM System/360, released 62 years ago.

The original architecture has been expanded to 64 bits and can (or should) run linux with older code in virtualization.

However, those mainframes are extremely high performance, high reliably, and high cost. Complete overkill for many companies, who can get away with much simpler/cheaper emulations of the System/360 from 3rd parties, or source code ports to more modern architectures.