Page faults are fine. Swapping to disk is fine. Being forced into a situation in which a suspended application is terminated is also fine. Sometimes things will happen that the OS didn't predict as well as you'd have liked and it takes a tiny bit longer as a result. This is all as true today on Windows 7 running traditional desktop applications as it ever will be. As the slower bits of computers (IO mainly) get faster, it just becomes less and less of an issue.
I think you're massively over-complicating how it actually works; the OS doesn't ever have to reason about execution, it simply keeps track of what state a process is in and whether it therefore needs to schedule any CPU time for it. Likewise it responds to requests for memory or IO as and when they happen (ignoring for a moment that memory managers often pre-empt expected future requests).
There is no difference in resource management for an OS in which all processes are conceptually always running than one where you've manually launched them all and just left them open. The OS already has to do the difficult balancing act of moving things in and out of memory, allocating CPU /GPU shares, balancing IO contention etc. It doesn't get any more difficult because you add other processes that are conceptually running but not actually demanding any resources at that particular time, because things not demanding any resources are a non issue. Even on Windows/Linux/MacOS today the OS itself doesn't actually care that a process is "launching" as opposed to just moving from the "waiting" to the "ready to run" queues.
It certainly matters to application developers, because it changes many of the assumptions about how it's acceptable for a process to behave and particularly what it means for a process to start or stop (there is a much greater emphasis on getting back to where you left off for example). And it matters to end-users because it relieves them of having to do things they might otherwise forget (ye olde "I didn't get the reminder because I forgot to start Outlook problem"). The OS itself (whether Windows, Linux or Mac) has been treating the world as if that's how it worked for a very, very long time now.