[Talus1]

I'm going to state a less optimistic view. I think the twins will slowly die a death precisely because they are twins. I don't think there will be any significant new investment in these models. They'll likely keep making them for a bunch more years, with some trim-only special editions to spice things up, but there won't be a "new" twin. They are not core product for either Toyota or Subaru. Subaru isn't likely to want to fund development of new major hardware, especially not RWD only, and Toyota has moved on to other things.

I think it is more likely that Subaru and Toyota will each try in their own way to fill the holes in their model lineups that would be left by the eventual death of the twins. For example, Toyota could try to "Si" the Corolla or maybe make a new, more sporting hybrid CRZ. Subaru could make another non-turbo Impreza RS stripper, with a focus on handling. (These are just illustrations of what is possible. I'm not suggesting they will happen).

Part of the reason that the MX-5 has been around so long is that it is a Mazda-only development and a core part of their business and their identity. Even they were able to defray the costs a bit by "selling" the platform to FCA (it wasn't a co-development, as the twins were).