I agree with you, as a general matter. And I expect that, given the size limitations of even a $350 set, there probably won't be much in the way an interior. But I'm game to try to modify it, and I think it may differ from the Super Star Destroyer in its ability to be modified.
I never had much interest in modifying the Super Star Destroyer because no matter what you do, the scale would be so off. I don't think it could "function" as a system-scale ship, if that makes sense.
In other words, regardless of modifications and its limited interior, the SSD is going to be a display model that just happens to have a small playset inside. And while other large ships also are not to scale with their movie inspirations -- of course -- there's something complete and coherent about them. Take the 2009 Tantive IV, with its bridge, interior, cargo room, etc. In an imaginary Lego world, you wouldn't need to pretend that the Tantive IV is actually a lot bigger than it really is. You could treat it as exactly what it appears to be: a large ship (almost twice as long as an X-wing) with a crew of 5. You could play with that spaceship in a way in which your other Lego spaceships would be to scale to it. The SSD, however, has difficulty functioning like that. Pretending that the SSD is not what it appears to be is almost required. It's clearly intended to be representative of something very large, not at all to scale with your other Lego ships. The exterior of the bridge is the size of a single brick. And the playset inside does not represent the entire crew that would operate the ship, not even in a system-scale, imaginary world.
Put simply, the Tantive IV could seamlessly be integrated, as to-scale, coherent spaceship, into an imaginary, system-scale world. It would be as equally at home in such a world as any of the three Space Police motherships. The SSD would not, and it would be difficult to modify it to make it fit in. The SSD doesn't exist in the same world as your other Lego ships.
So, to get back to what you said, I only buy sets like the Tantive IV -- sets that can be integrated into a Lego world, sets with playability and minifigures, sets that are toys, not mere display models. The new Slave 1 is right up my alley, and I'm thrilled that Lego is making UCS ships in minifigure scale. But while the helicarrier probably isn't going to have much of an interior or play features, I may get it anyway if there's any room at all to try to modify it. The intended scale isn't as far off as the SSD, and I can imagine that the carrier's tower could be modified to add a bridge (if it doesn't have one). And maybe you could include other interior spaces, like an engine room and crew quarters. And maybe you could land small system-scale fighters on it. If there's any possibility of turning it into something system scale, I'm going to try. Because helicarriers are cool.