People have been saying that for over a decade now, and their business is still going.
So is IBM.
There was a pretty interesting article in the Wall Street Journal a few weeks back about how IBM is flying under the radar, and doing well by not taking the hype bait.
It noted that 70% of all credit card transactions on the planet go through an IBM system.
It ought to be extremely easy now to use LLMs to transpile cobol and do the necessary reverse engineering to migrate off mainframes and aix servers. I think IBM will lose many customers.