When you combine retirements with decades of under-documented code doing critical functions, it's not a recipe for good long term success...
When I look at how mainframes work, and how "clouds" work, I wonder at how imbecilic IBM management (or one of their competitors) must have been to not capture that value in the first place.
http://www.winestockwebdesign.com/Essays/Eternal_Mainframe.h...
I'm not surprised at the mainframe's lack of success at all, the barriers to entry are extreme, meaning there's an inevitable lack of activity.
I've worked in environments which had mainframes deployed and even as an employee it was inordinately difficult to get any access to them.
Combine that with IBM's "enterprise" sales process and it's not a mystery that they lost the fight to attract newer systems...