The heart of the mainframe success was being first to make a real OS and real forward compatibility. The heart of the mainframe's continued existence is as you

The heart of the mainframe success was being first to make a real OS and real forward compatibility. The heart of the mainframe's continued existence is as you say, combined with risk aversion to changing software that currently works, and a lack of people who understands how it even works to know how to change it. But the heart of the mainframe's failure to expand is IBM's reluctance to join the modern world in pricing and availability. There is no simple way to become a mainframe dev because there is no simple way to get access to modern zOS. There is an entry fee. And it's hard to be excited about anything they do to the silicon when you know that you don't just buy the system, you're charged for consumption as if it were a cloud device. So, nobody with a wallet wants to run anything other than the essentials there, hence the platform never grows.