The cost of old systems sneaks up on managers
You have in house developers. They do a bit here, a bit there.
You now have a workable system that's meeting the business needs (these systems were built for *business* needs, not because Unisys/IBM/ICL/Fujitsu or MS had some new shiny bit of kit to sell) and you think you'd like to port it to the new language/platform that's going to take over the world.
And you multiple all those developer hours by the rate of developers in *new* environment.
To get back to where you are *now*.
Suddenly it does not sound *that* much of an improvement.
I've heard of some clever mainframe re-architecting tools (EDS came up with the dodge of putting it on a mainframe and have the developers email chunks of old IIRC COBOL in for re-work, then it emailed them the new version. No GUI, but a neat hack I thought).
And as for that old S360 Assembler code.......