Even if you say "March 14, 2021 is the grand final feature freeze for the Mainframe, and day one of the Cloud Initiative", it's not going to happen. You're not going to have a MVP for several years, and a full replacement for five million undocumented lines of COBOL for even longer. Production fires will have to be fought. Regulators will demand you change something, and won't be keen to hear "we're supporting it in the Cloud Initiative, which customers will see in 2025." So you're gonna be writing quite a few more cheques to IBM and your team of COBOL jockeys.
After 5 or 6 years, as you've unloaded most of the customers to your painstakingly built replacement, you can finally begin to scale down the mainframe staff and perhaps return/downgrade some of the rented hardware.
Given that, it may be less a put-your-own-head-in-the-guillotine move just to spend those years hiring in a few new graduates with the promise of "We'll apprentice you to handle our unique and obscure codebase, and as long as you show up 40 hours a week and don't take a dump on your manager's desk, you're going to have a reliable paycheque for the next 20 years."
My understanding is at least Java is well supported on mainframes. It's not the latest greatest language, but it sure beats ALGOL.
There are advantages to the mainframe, but there are disadvantages too. As a CIO your job is to consider both.