Yes, a complete lack of interoperability would make it really hard for criminals to target them.

However, a complete lack of interoperability would make it really hard to, you know, interact with other businesses and systems.

This isn't throwing the baby out with the bathwater so much as drowning the baby in the bathwater.

While that is a solution, I don't think it is the solution. Another non-solution would be removing all Internet access. Ransomware problem solved, a whole bunch of other problems created.

and what if employee brings infected USB drive and plugs it into computer?

Probably irrelevant. There would anyway be some number N of operating systems, and a number K of computers, and the number K will always be very much larger than N. So there would be a huge possible 'market' for these criminals, even if they targeted just one of the N operating systems, as long as the number of vulnerable computers is large enough.

(edit: removed some meaningless words)

Everyone who downvoted you would upvote articles about the risks of monoculture farming, without considering that the two are one and the same.