I'm sure they COULD save the old FITH if they wanted to, but the issue here is return on investment. Spending upwards of $10 million to re-model an existing ride only to end up with a somewhat updated version of that ride is not going to generate nearly as much hype or ticket sales as building a large new ride for $15-20 million, not to mention all the theoretical logistical improvements from having a bigger, better, modern space that is designed for the crowds SDC has now and not the crowds they did back in the 60's.
They could have simply replaced the lost river in place as well with new troughs and a new water supply, but Mystic River got them national attention whereas even yearly visitors may not have noticed an upgrade to Lost River.
There has been speculation that they want to build a new train station since the old one doesn't even fit a full train load and has always been a logistical sticking point. They don't even use the store space hardly anymore, and few people can get to the concession booth inside. I don't know though, they might just tear down Flooded Mine one of these days along with the train station and build a big new train station across the two spots. Anything is possible at this point. There are few left at the park who care about preserving anything vintage SDC. All we can ask is that they at least don't completely ignore the theme when they build a new ride. I still say I like the marvel cave tie in to Mystic River and that they did a good job with that one.