Re: Historical effects.
Life long Mainframer here!
Just wanted to point out for balance regarding mainframes in general (not limited to Government mainframes, which I know nothing about), my entire career, the mainframes I've worked on have been state of the art hardware. I've never seen an elderly mainframe, as everywhere I've worked has always kept them up to date, and the tech in each new mainframe is amazing.
There are no limitations forcing mainframes to be "old". Totally modern hardware, totally modern software (don't confuse software with a long heritage with being out of date).
You say pretty much the only mainframe manufacturer left is Fujitsu, which I guess is related to the mainframe architecture that the government IT uses, but again just wanted to point out that there are other mainframe manufacturers - if any company has a mainframe monopoly, it's IBM.
"Even if they do, and possibly migrate their mainframes to the cloud..."
The rush for Clou-tsourcing in crazy in my opinion. Seems to be top of every senior execs agenda to outsource as much to the cloud as possible, and yet most of the major incidents I see nearly always involve a 3rd party outsourcer, and comments such as "awaiting xxxx for an update". Nobody will ever care about your IT like your own employees - and mostly employees do actually care about the systems for the company they are employed by.