I think that in general the routes in East Sussex were rationalised too much, therefore I would have retained both the route North via Oxted and the Three Bridges - Tunbridge Wells Route (which also took in the reasonably sized settlement of Forest Row. All services could then have remained concentrated on the high level station.
The conflict with the fast lines at Three Bridges would have been a big impediment to Brighton and Coastway trains, and in the 60s the villages between EG and Three Bridges were hamlets at most. I personally would have closed the low level station at East Grinstead, retaining the line to the East with the sharp curve so that trains would have run Croydon-Oxted-East Grinstead-Forest Row. Forest Row-Ashurst Jn retention would have been unlikely although more palatable if Forest Row remained open.
Tunbridge Wells had a faster more obvious line to more preferable London terminals via Tonbridge anyway, I would have retained the Brighton-Tunbridge Wells direction link only, abandoning the north side of the triangle at Groombridge jn.
The route that would be doing the best today IMHO, however, is the Cuckoo Line. Heathfield and Hailsham are growing towns and providing alternative links for Eastbourne would perhaps free up space for Seaford trains to run back through to London.
However it is important to remember just how recent much of the development in Sussex has been. This area is a classic case where closures were justified in the 50s and 60s, but the right-of-way should have been protected as the future need was identifiable even back then, even if it might take 30 years to be viable again.