SDTS isn't really a "test hull." It's a weapons and sensors proving hull...we can mix and match the loadout to what we want to test. Recently that's been the SSDS suite, and now it'll probably shift over to proof out DDG Flight III combat systems.
And the land based labs used by AEGIS are mostly multi-purpose, there's no reason DDG-1000 or LCS couldn't make use (and DDG-1000 has previously for its radar tests).
AEGIS doesn't get away from fighting for hulls with the Fleet either. But it's "important" enough that they get a designated non-deploying...but deployable if we need to "test princess."
Right now it's JPJ, but before that it was Lake Erie (the BMD shooter that did the SM-3 satellite kill then got a CO fired for a shipboard goat when they went back to being a real operational asset again).
And that's probably pretty a good comparison to what DDG-1000 will end up doing...and eventually maybe like Seawolf.
And remember, we even had a big deck amphib (Wasp) for years as designated non-deploying F-35B test bitch.
So there's a long history of Fleet assets being relegated to test long-term.
DDG-1000's challenge is that pumping in a lot of R&D to get it up to par with a late model DDG-51 is a lot of money for a little operational return. At best we'd clone DDG-51 capabilities...which basically only gives us 3 more DDG-51 like ships. I'm not sure where they're drawing the line for its capabilities, but that's the tradeoff we need to balance.
As an at sea test platform to mature technology for use in a next gen surface combatant would be ideal. SDTS cannot replicate the electrical plant capabilities of DDG-1000.