I am not a mainframe guy, but AFAIK mainframes are very special beasts. Especially they guarentee very high aviability and have special hardware features to support this guarentees. So you do not use them in the first place if you are not more scared of downtime, than of hardware costs. And with this comes a specific very conservative style of software development. If it ain't broken, don't fix it. So a lot of the mainframe code is very old and will only be replaced, if the company is replaced by a competitor. And therefor compatible hardware will also stick around until the end of business operations.