I think Air, if I HAD to choose, it's definitely returned on investment, but not as much as Nemesis or Oblivion. 13 can be much better with more work done to the scenery of the ride, right now it's just a jaunt through the woods. Give me buildings to "crash" through, pits of oozing tar, anything.
13 could probably be scrapped the the area repurposed most easily. Getting rid of Air means shutting the Rollercoaster Restaurant as it all needs refurbishing. A hell of a lot of underground work too.
I hope that if Nemesis was to go they would retrack it with the same layout - do a Hulk. However I fear It would be viewed as a pointless investment, spending money (that could be spent on a new SW) on an 'old ride'. After getting through the pain of imagining a Nemmy-less Towers, I'd like to see how a different coaster would be in the pit. I'm imagining something like Taron, but I think a Mack Mega would work nicely as some sharp turns would be required and could work with the terrain well.
If AT got rid of Nemesis I don't know what to say, it really put AT on the map, it brings oodles of visitors, it's by far and away the best ride in Britain and it's a top contender for best EU and US coaster (IMO, it's World-class but appreciate not everyone loves it)
They would probably retrack it. I wonder what would happen if they updated it with a faster lift-hill. Can you imagine doing Nemesis but 5mph faster? Phwoar.