Let's go through each one, one at a a time:
It may be 21 years old, but let's look at the bigger picture. Drayton. Shockwave is by far the least maintained out of the 3 '94 coasters (If you don't know the third, you may as well leave the forum). Also, Nemesis is a B&M. It's not rough, it's still got that amazing throughput, the theming is still decent. Granted, it has it's bad days where it leaks rusty water onto the riders, but aside from that, it doesn't really have any problems. Most people rate it as their #1 UK coaster, and #1 Worldwide Invert, so it's obviously special. Although with the removal of the SRQ, it's gone down in my rankings...
Alright, it's just a simple dive machine that's not vertical, but logically, removing it would be a silly idea. The area around X-Sector would just look so out of place, with a massive tunnel going through it. Removal is not justifiable, especially not yet. Even later on, I can see it being SBNO until they get a replacement for the tunnel, but I highly doubt it'll be going soon. Easily has another 10 years in it imo.
I voted for Air to go simply through process of elimination. If it dissapeared, it wouldn't have too much of an effect on the landscape, just the two tunnels, which can easily be modified into something else. I feel like Air ruins Forbidden Valley as the theme doesn't fit. As for the ride itself, it's prone to issues. It's also quite slow and boring in my opinion, and now that Hero has opened at Flamingo Land, it's not the only type in the UK. Please ignore how bad Hero is.
When they got rid of Corkscrew to make way for Th13teen, they re-themed Ug Land. Removing Th13teen is just a stupid idea, especially because it takes up no visible room and does eat up a queue if ran well. Dark Forest doesn't need less rides, it needs more. Get a flat ride in there, or two. Don't remove the USP for the area, that's just poppycock.
Whilst I'm not a fan of the ride, removing it would be one of the stupidest ideas I've ever seen, on par with The Ultimate opening with OTSRs. It's what draws people to the park, and it's a world record holder. Now, as much as I dislike the experience, it's a HUGE hit with the GP. The branding is excellent and to this day, probably generates more income via merchandise than the other rides put together... (I said probably, don't be attacking me with figures and stuff). But in all seriousness, it's not going yet, or for a while. It'll continue laughing for many years to come, and who knows, maybe it'll outlive the B&Ms. I doubt that though...
I'm the one with the horse mask that likes Vekoma...