The only reason to buy a mainframe is legacy apps. However the hardware is not special and neither is the operating system. Linux can run for weeks without booting. My mobile phone running android does its all the time.
I grew up with mainframes and my first job was a large mainframe site. I then moved onto as400’s a machine I still love. However I cannot see new customers moving onto a mainframe. With ultra fast SSD disks on a PC server the advantage is not there anymore. I doubt IBM mainframe business will die but it will become a smaller and smaller business.
Eventually IBM will leave this business, either by selling out or reduced investment.