As Always it depends
If you are talking 20+ years ago you are correct about the power costs. Early in the 80's there was no longer a need for power/water cost to be high. All Mainframes that I know of had a divorce a long time ago from water cooled.
The latest nainframes are indeed a bit stingy on the power needs.
Some operating systems run at 100 percent cpu busy 24X7 and they hum along as there is almost never a strain to put more work on. You can't say that about any INTEL based OS. In Fact if you run at less than 100 percent chances are your work is going to take longer. Long long ago the max memory you could put on a M/F was 8 MB now its 2GB (and might be more). Can you shut down specific cpu's on INTEL for maintenance while the system continues on without a reboot, no. Can you add memory on the fly, no you cannot. While you can do it to a limited extent on a M/F you really have to "reboot" to take advantage to some extent.
Yes the MF may seem old and its there so I have to put up with it, but indeed it does run a majority of Fortune 500 business's still and there is a lot less need for scheduled IPL's than there ever has been before. When there is an outage, you can generally find out what caused it and get IBM to fix it reasonably quickly.
Someone mentioned the MF as being secure, all I can say is I agree 100 percent *AND* if you find a way to bypass security and IBM recognizes it as an issue it is given a high priority within IBM to find a fix and send the fix out quickly. Can MS or MAC say the same?
IBM has a solid reputation for Security and relability of fix's. For the last 25 years the broken fixes IBM supplies has gone down to a small 1 digit number) as they first test internally and then ship the fix out to interested parties and as soon as they give the all clear only then will be fixes be made generally available..
IBM M/F's have also decreased the number of hardware problems to a small percent (it depends on the hardware that is talked about).
Approximately 30 years ago we had just gotten in a new M/F and the first day it went into production if failed about 6 times. The second day it failed 10 times. The third day we had IBM flying in people from Poughkepsie on jets and it we kept the system up for tthem to get a solid idea why the system kept failing as none of their tests hsd shown any problem. One of the system guys wrote a guarenteed never to fail program and we were able to prove to IBM it was absolutely a hardware problem. Then it took less than four hours to come up with a solution and they replaced a tri-lead into the high speed memory. It worked and never failed (for that reason again). One wire was 2 inches to long.
Never have I seen such good response from IBM on an issue. Somewhere around early 1990's
IBM started to go down hill. They laid off all their really good people it seemed like and their service has not kept up their previous level since then. It's still good mind you but far from what it was.