You raise an interesting point but IASW wouldn’t be the best comparison. Much of what makes the subs so iconic is...well, the subs. Small World has copies in every Castle Park save for Shanghai, and it shares its relatively common ride system with Pirates and a few other one-offs. Only the two original castle parks got Submarine rides, and both were seemingly closed forever in the ‘90s. It was a miracle that we got our subs back from the dead. Even considering the crappy Nemo content, to be able to take a Submarine Voyage at Disneyland again is a triumph of nostalgia. Not everyone would agree that’s enough of a reason to keep the subs around, but it does make it a different case than IASW and other, less novel attractions.
A better comparison may be Universe of Energy (RIP). It too had a novel ride system. You could argue that it’s the same basic system as the recent trackless dark rides, just as SVTLS/FNSV is the same basic system as Jungle Cruise. But just as the subs have the novelty of putting you half-underwater, UoE had the novelty of being a full-scale traveling theater. That novelty was a source of fondness for fans, even after the ride was downgraded to Ellen’s Energy Adventure. When GOTG was announced, many fans shrugged it off, quick to remind us that Universe of Energy was already butchered a long time ago. It seems to be the same point you’re making about the subs, and it’s a very good point in both cases. But as for myself and I suspect some others, I really miss it and wish Epcot could just still have a traveling theater in some form. Even a crappy traveling theater. Even a crappy Guardians of the Galaxy traveling theater! The traveling theater itself, as a format, was so novel and grand. So EPCOT.
I’d feel the same way if we were to lose the subs, for exactly the same reasons.