Not true. If you view the network as the computer, then the network is now forever winning.
What is Google's or Facebooks uptime? Their computer is their cluster. Those clusters are never going to be powered off ever. They can upgrade and replace the entire cluster without being down. If you have a cluster of 10,000 nodes, turning off 100 or 1000 is not downtime, so longer as it's up and serving. No mainframe ever will run for the next 100 years. It's a possibility that a cluster built today could be running for the next 100yrs.
99.9 percent. Cloud services from Microsoft, Google, Amazon typically have only three nines or less for paying customers. They also seem to have same level of uptime in their own services.
But these are not "firm guarantees". For example Amazon just gives service credits after downtime exceeds what is promised. They "undergo unscheduled outages" from time to time. Whole regions are down at once.
Remember Amazon's Prime Day outages? It's not good for business when tings go down.
It's harder to get mainframe like uptimes for OLTP simultaneously with mainframe like IOPS.
And I bet all that costs much more than an IBM. Not a computer, the company.
Single-thread performance is also pretty high, so it'd only lose in the aggregate CPU benchmarks.