Re: POWER to the people!
You're mixing up AS/400 with the hardware it runs on. I agree with what you say about AS/400 (everyone still calls it that, right?) but in reality they could have made it run on x86 or whatever if they really wanted to - that's one of the main points of the OS - a virtualisation layer between code and hardware.
The cost of AS400 is something above and beyond Power. After all Power running Linux costs less than Power running AIX costs less than Power running i. All on the same hardware. Or at least it did last time I looked - feel free to correct me if this is no longer the case.
A lot of enterprises do run AS/400 as you say - I have seen plenty of instances. There are plenty of them that see the value of them, as with the mainframe, but equally plenty who have either got themselves off, or are desperate to.