The trouble with restoring too much of the accessible parts of the Towers is that delinquent kids WILL just smash things for a laugh. I do sympathize with the Towers on that one. If they were going to replace a load of windows to their original design it would cost a lot of money and then they would need staff to 'man' the building pretty much permanently to protect the investment. Realistically the budget just isn't going to allow for that. I'm happy enough with having the safe parts of the building open to explore during open season with a few projects going on here and there. Remember, they spent a fortune on restoring the gallery range including the Octagon for the Hex attraction, restored the House Conservatory, restored the Music Room window and are currently in the process of restoring the massive Banqueting Hall stained glass window and the Gothic Prospect Tower. They also had to do a lot of work on the Round Tower a few years back as well just to keep that from falling down, which would probably have taken more with it! There's also stuff like restoring Her Ladyships Oratory after the tree fell on it plus other stuff that I've probably forgot off the top of my head whilst writing this post.
I think they do a reasonable amount of work on the Heritage side of things, slowly but surely. Like others have said though, ideally we would want a lot more restoration, but it's just not realistic that it's going to happen due to cost. By the way, still no reply from English Heritage but the Towers appear to be open at the moment so no rush to fire off another email to them.