I can't imagine Oblivion going anywhere. Closed for half a season to retrack in its entirety, yes, but not removed for good. It's their arguably most iconic attraction, and without it, what are you going to use that giant hole for? It would probably cost more to refill the tunnel than to retrack the coaster, and there's not really much else you can put there due to the entry and exit angles of the tunnel.
Nemesis is iconic as well, but it sits in the open, is easier to tear away and easier to replace. I mean, there's a bazillion things you can put in a crater/valley as opposed to a tunnel. Financially speaking, Nemesis has done its job. Its income has surpassed its initial cost, while its novelty factor is wearing off. People are no longer going to Alton "because of that cool upside-down coaster", but to ride "that awesomely scary coaster and the one that goes 100 mph", and do Nemesis as well while they're still there. And I guess soon, it will be more expensive to maintain, eventually critical parts will need replacing, and it might be more economic to tear it down and build something new in its place than to refurbish it.
Rita is a different story, though. While we regard it as a mediocre ride that makes no sense theme-wise and is just dumb ugly compared to a lot of similiar coasters, it does deliver a fast ride, a thrilling acceleration and is easy to market due to these raw numbers. She is also much, much younger than her brethren in the Forbidden Valley and X-Sector. While perhaps mechanically more challenging, the cost of maintaining it can be justified for at least a few more years. Perhaps to us enthusiasts' dismay, but after all, it's money that makes the parks go round.
There's no point in taking 13 into the equation. It's just too new.
So yeah, I'm guessing Nemesis is the first to bite the bucket (or whatever). It's the oldest of the lot, it's the easiest to replace, and it doesn't have the same draw factor as it used to have. It will probably stay around for a few more years, though.