There is no need for a realtime simulation. What it feels like centuries can be a fraction of second for such hardware.
With this kind of simulations you can obtain, in theory, "what if" universes where alternatives solutions and technologies can be created. If you can observe them and the simulation rate is faster than your own time, it can be a rewarding experiment.
I don't think you can simulate your own future, even if you accept #3.
Either #3 is false (and there are no simulations) or #3 is true (and the vast majority of sentient beings live in simulations, which means those running the simulation are probably simulations themselves.)
Yes a computer can run a simulation of an alternate version of itself, but then the speed must necessarily decrease as you go down the hierarchy (as you will see if you try to run one vmware vm inside another.) Or the simulated universe must lag behind the one containing the more powerful (thanks to Moore's law) host computer.
Edit: unless you have an infinitely-powerful computer like in that story. But then you could argue that every theoretically-possible simulation must be being run by someone somewhere. Good for us even if our society never discovers the infinitely-powerful quantum computer (at least we don't need to worry about being switched off, even if our creators die or their universe collapses.)
With this kind of simulations you can obtain, in theory, "what if" universes where alternatives solutions and technologies can be created. If you can observe them and the simulation rate is faster than your own time, it can be a rewarding experiment.
About universe observing, this story is cool: http://qntm.org/responsibility