Datacenter-scale compute on non-proprietary platforms (e.g. leveraging OCP standards and best practices, which have been developed w/ significant input from FAANG and 'cloud' vendors) can easily tackle any mainframe workload. The implied path is obvious.

That being said, it takes a massive commitment and pretty much a rewrite from scratch to achieve a jump from mainframe to modern DC. You need to embrace individual node failure and that's not something you can just hack in.

AWS, Azure and GCP are all proprietary, and all are producing larger and larger servers and VMs. They are basically rentable, virtualised mainframes at this point

Most of us didn't consider writing cross-platform software back then and the few who did were writing software for Unix, not Windows, and that because there wasn't a single dominant OS (Unix was a fuzzy target) and binary architecture you could write for, so you had to think about portability.

Oddly enough, there was a lot of cross-platform work on games in the 80's because the home computer market was very fragmented.