Concrete over Asphalt? Would you do it?
Concrete isn't for resurfacing. It's a structure unto itself. So you need to likely pour at least a 4-5" slab for it to work as new garage flooring. 

So that means your current flooring would have had to sink 4-5" already. The catch with that is that for it to sink (at all, really) it means it had a poor base to begin with (or no base). 

As such, I'd say this is a bad idea.

I think the arguments for pouring on asphalt would be that if the asphalt has remained in good condition, it's likely in no small part due to the good base it had.