Not all computers will ever be as powerful as personal computers. There's a huge market for computers with limited memory and processing power, because they're embedded in other devices too, and these devices need to be cheap. Where do you suppose all those 8-bit microcontrollers, which have processing power roughly equivalent to the C-64 I first learned to program on twenty odd years ago, are used? They're in your cellphone, your watch, your air conditioner, your washing machine, in your car. Even if memory prices for personal computers fall to levels such that everyone could afford a personal computer with 10 petabytes of storage (which frankly, I don't think this will ever happen, given that the size of the entire Internet seems to be in that neighborhood), garbage collection will still be very much useful, if only in the embedded systems that are pretty much everywhere, even in subsystems of personal computers as well.
