There are only so many GSO satellites you can have at once, because of the way they're licensed in certain slots. And given that they last 20 years each, you're only talking about a couple thousand a century. Likely not all at the same altitude and not necessarily orbiting exactly around the equator (I don't know what 'rules' there are for how to park them) You'd need a lot more time than that for them to become a problem. Heck if they are all orbiting aroudn the equator it is pretty easy to avoid them - when leaving the Earth go don't leave exactly along the equatorial plane. They probably never do because their intended destination is probably never exactly along the equatorial plane, so they don't even need to consider the GSO & parked satellites because the craft isn't coming near where they are.
Like I said, LEO is much worse problem today and it will be far worse as time goes on. We'll probably have to think of a solution for it before 100-200 years go by, meanwhile the parking/graveyard orbit for GSO satellites will continue to not be any sort of issue for anyone.
