I don't buy this. Many of our critical systems are on PC architectures. Mainframes don't have some magic sauce, well designed distributed architectures should offer enough reliability.
Edit: reading comprehension fail, please ignore my comment :-)
Why waste your money on a single rack-mounted PC when you can buy 40 cheap cellphones running Android and network them together. That'll probably provide greater reliability.
The obvious answer is the same as why some use mainframes over PCs. You can't easily convert all workloads running on a rack-mounted PC to a network of cellphones. Similarly, you can't easily convert all programs running on mainframes to running on PCs.
Which is what I was alluding to with the last paragraph in my upthread comment. There are mainframe use-cases that are genuinely entwined with those hardware platforms. A mainframe isn't just a fridge-sized PC.