A lot of it is down to Alton really, and what they want from the space within the park. I mean look at Corkscrew and the Revolution (BPB) both constructed within a year, by 2008 one was a wreck, the other has recently been given a new opperational system and a complete overhaul that should see it though another 15 years service.
That is not a dig BTW at Alton's maintance, they could have done the same to Corkscrew had they wanted to, clearly the cost to do so, given it would see little if any return on the taking at the gate clearly lead them to believe its removal and the introduction of Th13teen was the best option. For the park maybe, I'm not too sure about the coaster community as a whole, most seem divided on the matter. However given the percentage of the market we make up, Alton cant be blamed for not paying too much attention anyway!
I'm sure Nemesis will be around for a long time yet, its not as if B&M are a company of yesteryear, parts for inverted coasters are still there should Alton require them, I'm certainly not expecting Nemesis to bite the dust any time soon!
Ash