I know its best to ignore what Cast Members say, but what one Monorail driver told me the other night stuck with me because I can relate to it from a worker-to-corporate level.
I asked if she thought the subs were going away/coming back. She said she honestly has no idea, and that NO cast member really knows OR will know until a few weeks before it opens/an announcement for a new ride. I work at a furniture store, so of course people are always asking me "WHens the next sale/when does it end" and similarily, we dont know until right before it begins/ends. Obviously its an entirely different corporation I work for and its an apples to oranges comparison, but I can believe that over some cast member saying "Yeah, they're coming back" or "Nope, star wars is taking over". DO you really think DIsney is going to "only tell certain CM's the truth on their plans" and not others? Nah. None of them know exactly, they just know of the "Rumors". She also commented it would be unlikely because of Finding Dory coming out soon, she speculated (using that word exactly, Speculate) that they might be adding in new elements from that movie to coincide with Finding Nemo Submarine Voyage. I think she's on the right level, thinking wise, because that makes more sense to have it open with new "finding dory" elements, as Disneyland celebrates its 60th anniversary, then to have a huge construction site during the middle of the 60th anniversary for a Star Wars ride. If anything, they'll wait to see how Episode 7 fares with people via Market Research to determine if its popularity warrants a ride adaptation.
Commenting on the Rocket Rods comparison, the fact that the entire lagoon has been drained and ONE person is seen doing work is alot stronger of an indication that something is happening. I remember going to the park seeing that "Rocket Rods will re-open" sign and seeing ZERO work being done on the ride. Furthermore, even if they were going to remove it, we wouldnt know at this point. Until we start seeing each submarine disappear, and/or being removed from the park and shipped away on the 5, any speculation on if they will return/be removed is just that. Speculation.