At this point no one is buying into mainframes are they?
So they are just selling to the existing pool of users for which the price just has to come in under the cost/risk of rewriting all their ancient stuff.
Someone has to be buying mainframes. Otherwise, there's no point in continuing to develop them. Shareholders would revolt.
Someone has to be buying mainframes. Otherwise, there's no point in continuing to develop them. Shareholders would revolt.