Banks, insurance companies and airlines. The amount of only-documented-in-code corner cases and custom business logic is immense (for the really old, up to 60 years!) which means rewrites are immensely difficult and expensive (and prone to failure), the amount of government regulation and the fines for when things go south (aka people lose money) is equally massive, and the companies are ruled by beancounters by definition which means they will, as long as IBM has mainframes and support for them, go with keeping their mainframe stuff to "never touch a running system".
That is part of why fintech start-ups are eating the old banks and insurance companies alive... they can design proper systems from scratch without having to take care of decades of accumulated cruft that needs to be migrated and especially they don't have to retain horribly outdated and complex business logic, workflows and processes that would take old-school companies billions of dollars to re-train their staff - almost 2 million people in 2019, per https://www.statista.com/statistics/250220/ranking-of-united....
Oh, and speaking of the rest of the PC market, another natural monopoly on the other side: servers are dominated by Linux.
[1] And historically by Intel, ergo Wintel.
?? PC (Desktop) OS are certainly dominated by Microsoft, but I don't see any one player dominating the PC Market. There's definitely companies that dominate different PC components (OS, CPU, audio, network, etc)
Anyway, I know that the mainframe market is pretty healthy, but who are the clients?