It's the sheer scale of Oblivion and, as Crofty said, the way you're so close to it.
Nothing really compares to the noise and sight of it as you enter X-Sector. It's massive, and the trains roaring into the hole and vanishing, with the screams is actually quite stunning. I still know people who just point blank will not go near Oblivion, despite doing pretty much anything else anywhere in the world.
It's got a lot of life left in it yet, and a lot more newcomers to scare before it leaves X-Sector. Remember, the nearest Dive Machine is still a long way off and out of the reach of most visitors to UK theme parks.
I am one of those people just too scared to go on it. It just strikes me with fear. I think it is an awesome ride,the whole buzz it gives off as you enter X sector is something a lot of other parks must be in envy of. I have been on what a lot of people would term more gut wrenching coasters throughout the world, but just can't pluck up the courage for oblivion.
I cannot see it removed for years and years, it would cost a fortune to do so at anytime. Its not as if it could be removed and sold on to another park easily as its a total one off unlike rides such as corkscrew or Black hole (not that I am implying they were sold off to other parks) which could easily be rebuilt elsewhere.