@1npc They are in slow decline - neither Z nor Power will ever see the kind of growth that the real cloud players are experiencing, rather the reverse. There's no new logos for either one. P will decline faster as it (barring I) has a slightly newer code base that can better be ported to modern systems. Z users are stuck on the platform because of the massive code base that's currently prohibitively expensive to replace. No new logos - no growth.
I do think you have an interesting point though - Red Hat might be the middleware glue that let's the ancient systems talk to the modern world...essentially a modern WESB. That's actually not a bad preserve the stream strategy, and might explain the buy. Unfortunately, that market isn't remotely going to compete with the explosive growth of AWS/AZ/GCP - at best it'll flatten and postpone the continuing decline.
@1qrs Take a look at the mainframe cycles...yes, there's a cyclical bump, but the long-term trend is that those bumps are getting smaller.
In the end, IBM was trumpeting 5% cloud growth, on top of 2% market share. Amazon and Azure were getting 30-40% growth on top of 30-50% market share. If cloud's the future, IBM is already irrelevant.