In it's current condition Nemesis (As just a coaster) will probably last another 15 years or so. More than 20 I think is pushing it unless there are changes to how the park care for their rides.
In theory, if the park were willing to throw the money at it Nemesis could be made to live on for many many years, replacing components as they are needed. B&M's guarantee covers defects from wear and tear, and manufacturing defects. However, as with any guarantee there are conditions, and B&M's are incredibly tight! They'll void it if they think the park have neglected to follow the care and maintenance procedures laid down by B&M in any way. I believe one of the Six Flags parks found this out the hard way with one of their B&Ms
Nemesis' main enemy is itself though! The ride is too forceful in places, and literally wears itself out. The helix is one of the main culprits, as it places so much strain onto both the track and train. I remember hearing something a while back where most Inverts needs their wheels changing every some many months (I think it's 12 or something similar) but Nemmy doesn't even last half as long due to what they are subjected to every day! You only need to take a look at the helix to see where it's had to be patched up over the years (Those black looking burns aren't paint and intentional distressing. It's weld and cut markings!).
Just last year the ride was closed for a day or two while the park repaired a crack in the loop.
The problem is though, on a lot of rides the parks would just weld it up and have done with it. With B&M it's not that simple. Strictly speaking if I remember rightly any form of defect like a crack or break
has to be reported to B&M. They will then assess the damage and recommend the repair needed. Sometimes the smaller cracks can be done in the park, by their staff, and B&M will then inspect it and sign the repair off as satisfactory. Sometimes though, B&M will have to perform the repair, and at times like this track can start disappearing for a while, whilst it's sorted out.
There have been a couple of rumours that Nemmy has had track out in its life (Wether they are whole new pieces or repaired old ones which went back in is unknown). The helix, station roll, and loop have all been suspected as being removed at some point, though as I say, it's still a bit of a mystery which were (If any). I'm sure about 5-6 years ago though there was a photo floating around briefly which showed a segment of track removed...
One thing which is certain is the footer work which was done a while back. The area surrounding the loop had to have some of the footers recast and strengthened, as the stress from the ride, coupled by the location and surroundings of them had caused a lot of wear and tear on them.
Poor Nemmy doesn't do herself any favours! Apparently B&M have been quoted as saying they will "never build another like it". Shame really, but at the same time I don't really blame them!
If the ride is going to live on though I would hope that the park are going to start throwing a bit of cash at it. Not just for things like a clean and theming repairs, but a total service from top to bottom of the ride. Nemmy is considered to be one of the 'rougher' B&Ms out there, and age has taken its toll on the ride. No level of build quality can ever completely prevent age setting in sooner or later.
In fact, I'd say all the B&Ms at Towers could do with a good once over to bring them all back to their former glory. Oblivion could do with a good clean up of the whole ride and a lick of paint, and Air wouldn't be hurt by a fresh coat of paint, or at least a good jet wash. Apparently, B&M have offered to upgrade Air to improve reliability and bring it up to date with the other fliers out there. It's probably just a case of if Alton are willing to pay for the update (As there's no form of defect this wouldn't be covered by the guarantee).
With a lot of TLC, and a plan for future renewals and replacements Alton could keep Nemmy running for years to come I would hope. The sad thing is though, when its time comes that will more than likely be it. Same for the other two Alton B&Ms. The nature of the ride would make putting it anywhere else an absolute nightmare. It was designed for that plot, and that plot only. It's not as easy as with a car park coaster where you just pick it up and plonk it somewhere else. You would practically have to rebuild all of Nemmy! Plus, can you imagine how bizarre it'd look on a flat plot?! The station would be right up in the air! I already think Blivvy's mirrored clone looks odd enough
The day Nemmy finally goes will be a dark day in theme park history to say the least, and I sure hope that when it finally comes Alton will give it a true send off. Until then, the least the park can do is work their damnedest to keep the ride running for at least another 20 years.