All good points -- I was definitely one of the ones who bought the expansion on the day it came out :-)
I think the 1.875m parts are different, though -- mods can already make their own 1.875m parts. My point was (and I'm not a mod author so I may be mistaken) that the new engine plates seem to function beyond what's possible for a mod in the base game. Or maybe it's *possible*, but making it *standard* simplifies mod code, enables sharing, and encourages mods to extend the system even further in new ways. Take the new variant system -- it was already possible (I think) to accomplish similar part (or at least texture) switching through mods, but by standardizing a variant system in the base game and exposing it to mods, people can build from there to create even more amazing mods without having to code and maintain that themselves.
So yeah, I definitely have no qualms paying $15 extra for all the time KSP has given me -- I was just thinking aloud about the difference between isolated changes (like new 1.875m parts) that make sense for a DLC, and lower-level, fundamental changes (like the variant system) that make sense to expose in the base game because of the community effects, then still make specific use of them in the DLC. I could see the engine plates going either way.