The risk mitigation factor from large vendors comes not from performance but from reliability. You can trust that old mainframe code will continue to be supported for many years. You can trust that your COBOL workload will be continue to run, and that the 20 years of updates, patches, and tweaks have made it reliable and predictable in a way that precludes rip and replace in all but the most extenuating circumstances.
IT budgets tend to be governed by an 80/20 rule - maintenance vs innovation. Sometimes, its more of a 95/5 in practicality due to shrinking budgets. My experience says that the innovation side is drastically shifting away from these big vendors, but that the maintenance side has a lot more riding on it still.