Datacenter and back-end apps simply don't fit on a single machine anymore. Every app of reasonable scale is probably a distributed system of some sort. That, and there are a new class of "apps" (or, more precisely, datacenter services) that were built to operate across fleets of machines from day one, such as Spark, Hadoop, Cassandra, Kafka, Elasticsearch, and so on
On the other hand, a single machine can get pretty big today, for example http://www.supermicro.com/products/system/2U/6028/SYS-6028U-... is 2u, has 12 drive bays and can have 1.5 tb ram. You could do 24 bays if you are using 2.5" drives (most ssds). A lot of complexity can be avoided by getting many fewer, but bigger machines (doesn't help much if you need massive CPU though, but intels architectures are getting better every cycle)
The problem with this is that singular systems require a lot of engineering effort and produce interesting side effects when you attempt to make them reliable.
and sharing state between them? or ensuring that only one is doing things when only one should be doing things? (STONITH needs to be implemented somehow)
the performance penalties you enjoy if you go down the shared disk path (which is still going to be a fun failure when it does fail)
Distributed computing is now the norm, not the exception, and we need a data center operating system that delivers a layer of abstraction and a portable API for distributed applications.
And then we'll need an OS for handling multiple data centers. Its gonna be a crazy day the first time someone realizes they accidentally rebooted 5% of all servers in the western hemisphere.
