Maybe I am prejudiced but I have the feeling that mainframes get sold to shops with a lot of money based on the salesman's skills and, well, I wouldn't be surprised if there was a kickback here and there, to oil the old engine. Not because they are the best tech.
That may be a bigger obstacle to people getting in that area. You can make money in non-corrupt companies where your career won't get stuck.
I bet the kickbacks are like any other enterprise IT sales process: wine & dine, golf and box seats. Except the restaurants and courses are probably much nicer. Decision makers have pretty strict policies to follow that restrict gifts over a certain dollar amount but invite a bunch of employees to a box seat at a sporting event you already paid for and how much that gift is worth depends on how you measure the value.
That may be a bigger obstacle to people getting in that area. You can make money in non-corrupt companies where your career won't get stuck.