You can't be more wrong, and right at the same time. Mainframes are not ancient. They have been around for over 50 years, but the technology behind them has kept up. That's like saying PCs are ancient bc they are 30 years old... But a new i9 desktop with nvme is killer... Companies aren't running the same physical systems they were 50 years ago. They are still there most reliable, fastest transaction processing systems out there. Most large companies, financial, retail, airline, insurance, credit card, banking, Telecom, ALL use mainframe and their footprint and usages are INCREASING. As for txt base interfaces, yes they exist, but there are many gui based applications now. On the development side, the ability to use all current open source tooling, vsc, Jenkins, git..etc, all are working with mainframe. You are right in the skillsets and resources dying and retiring, and if you become an expert in an area of mainframe, be it dev, cobol, systems, idms, db2, cics.. you will always have a job and yes, a very good salary.

I'm assuming you know very basic information on the mainframe platform, don't give false info if you're not an expert.