It's funny, people frequently assume this would be true but reality doesn't really bear this out. It's typically pretty average to even below average which contributes to the talent pipeline problem.
The other side of it is that its not the technical stuff like "knows COBOL" that is so immensely valuble. Any average dev can "learn COBOL" but that's not actually the valuable thing. The anecdotes of COBOL programmers coming out of retirement for 500k/yr contracts has little to do with COBOL, but their accumulated institutional knowledge of the giant ball of business logic encoded in that COBOL.
If these banks and other institutions actually did write fat paychecks to young mainframe programmers the demographic problem they're facing might not be so bad.