@1hal google cloud is a late entrant and already gaining serious ground. They are emerging at most customers as the cheap/commodity option, especially for companies that compete with amazon in other ways. They will be a credible third player in <18 months.
Ibm has nothing that google would want to buy that they couldn't get elsewhere at a better price with more current technology. Ibms software is dying from neglect and yesterday's technology, hardware is irrelevant, services more expensive, lower skilled, and less profitable than competitors due to overhead and all the cuts over the years. Research is a shadow of what it was. Google is Working to k--l red hat and openshift by moving containers to direct hosting on hypervisors and donating the code back to the community.
As for z, it's a cash cow because of vendor lock in (and customers know it, and many resent it). But there's no growth there, no new logos, and companies would drop it in a cold minute if they weren't stuck with the legacy business applications. The myth that z drags distributed software is a fig leaf covering massive internal reversals because the reps pack c-ap into the ela that the customer doesn't give fertilizer about. It's almost as bad a joke as SO dragging software and hardware revenue.
IBM is about managing the declining business as long as possible. That's likely measured in decades because of, as you say, the monopoly lock in. The only hope for change is red hat ceo taking over and jettisoning most of the existing stuff - including mainframe - and keeping and stoking those small embers of innovation burning in the corner.
Put it another way. I hope the red hat ceo has a valerian steel blade to take out the night queen and the rest of the walking dead,