For me, it's not so much about increasing overall park capacity (although that is needed), it's more about adding additional things that I would like to experience. If an Indiana Jones ride were to replace the stunt show, for example, there's no guarantee that Disney would add any kind of secondary attraction along with it. If it were to be added with the stunt show, we have another net gain for the attractions lineup. There's a potentially-usable plot out behind the stunt show heading into the parking lot, is there not?Studios doesn't really have any "expansion plots" that they can use without replacing. The only one I can think of is near BatB, but it's not incredibly large (although another ride on Sunset is needed). I do agree that the next project should be something in the Animation Courtyard area and use up a lot of that backstage office area. Relocate the offices elsewhere and turn the land into guest space and become that much closer to connecting to the RNRC courtyard.
Replacing can put space to better use though, as seen with SW:GE and TSL. Indiana Jones Stunt Spectacular takes up A LOT of land. You could probably put multiple rides/an Indy mini-land on that space. Just to show how it would increase capacity, i'll lay it out here...
Indy has 6 performances tomorrow, with the last being at 6:00 PM. If the theater fits 2,000 per show, that's 12,000 people it can handle on a given day. Not really that many when you think about the entire course of a day. If Disney added a ride there that operated from open to close (7-10:30 tomorrow) and let's conservatively say the capacity is a low 1250/hour, around what Slinky is. Even at that number, it's seeing more guests per day. It would see roughly 19,375 during tomorrow's operation, an increase of over 7,000 on IJSS. If an Indy ride were to see higher numbers, maybe around 1500, it would see 23,350. If it were able to do closer to 2000 - say 1800, it would see 27,900 guests based on tomorrow's operation.
So while replacing may not be something we all want, it can help, especially if what you're replacing is a show. And that's not even taking into account that there may be leftover space for a second ride. Using Saucer's supposed 800 per hour, that would add 12,400 in capacity over the course of a day just in itself. So if we got a major E-ticket (Shanghai Pirates tech is the rumor) and a smaller scale ride, you could replace what currently is only giving you 12,000 in capacity over the course of a day and bring it up to around 40,000 or maybe even higher.