Mermaid utilizes screens, as in a television/monitor with rocks built around it. This is kind of interactive museum-ish, not the level of an E-Ticket, or even a Disney attraction, and not at all the same as the Projection Tech used in Nemo!
Nemo uses projections where the only "fake" thing is the character, the background is real. Not so with Mermaid as you instantly accept the screens as being screens. When I first rode Nemo it was pretty cool, like these little fish characters were swimming around real corral and stuff. Nemo's projections (not just plastering rock around an LED/LCD monitor) takes a lot more creativity and work, and produces a much better effect.
I feel bad for folks who can't experience the old Subs ride, I feel your pain as I really miss Country Bears Jamboree, which only got a couple updates and was run in one theatre at the end, making wait times jump from 15 to 30 minutes at max, or something like that.
But the Nemo subs are still popular and guests are waiting for them like they'd never wait for Country Bears. And unlike Country Bears, the characters are still wildly popular, and with Nemo 2 coming out . . . spending money to keep the Subs open is almost free advertising considering the promotion for Nemo 2 will probably be $200 million . . . the cost of two Mermaids, easy, maybe three.
I can't see the Subs leaving until Nemo 2 DVD and merchandise has been forgotten and is off shelves, probably three or fours years after Nemo 2 comes out, that would be in . . . 2020, meaning that the subs are good for at least a 13 year time span, probably a lot longer. PLUS, Disney doesn't need to make the mistake of Country Bears, they can add new stuff to Nemo (like new characters from 2), thus extending the shelf life of the ride to at least 2025 if they add new characters/scenes post Nemo 2 in 2018, another seven years . . .