DHS certainly has expansion space, but agree that it is not readily accessible. But I don't think that they "failed to consider" future expansion, just that they didn't make an effort to prepare for it. They just kicked that can down the road.
We know that had specific pads available in Galaxy's Edge for expansion set aside (for the restaurant and for another attraction). And there's still a bunch of backstage area on the west side that can be used for expansion, they would just need to relocate those functions currently housed there (but that was true before the DHS revamp, so it didn't change or worsen at all).
When the time comes, they basically just need to tear down the Animation building including the offices (+whatever they want to do with Mermaid) and connect that area to the RNR courtyard to make the most logical next location for a big expansion.
Separately or additionally, they can remove the IJ theater and the use the Echo Lake area to build into the parking lot. which could be a very large footprint if desired.