I wonder if at some point it begins to make sense to just run buys for addon, complimentary/complementary sets that are not full kits.
Yes... and no
It would make it a lot more affordable for established members of the community to get stuff they want, but it would also mean that people who don't own the compatible base set (new people, people who didn't like the set back then, etc.) would have to pay aftermarket prices (which only the minority would be willing to do) or mix it with a different set they own (which might not colormatch as well, also turns two things into one) or just skip it in which case they don't contribute to MOQ. Remember, the people that have thousands of dollars in GMK caps at home are a really small minority.
It also doesn't make a lot of sense either way, for example: I do own a set of 9009 which would be color compatible with this if it was an alphas-only run, but that would leave me with an additional set of alphas that's basically useless. If I'd "just buy a modset" for those alphas I'd still end up with a lot of missing keys while probably spending more money in total than on a full keyset.
That's not entirely true though: you can buy used (shiny) classic beige or black, or "Dolch" doubleshots with old boards on eBay (or some other less common colorways like Desko or WEY caps), often for much less than a brand new GMK set of the same colors. Or there is Originative which has been selling classic beige (and other colorways as well as accent kits) for
many years. If you're talking about some uncommon custom colors, then yes, buying full sets is preferable. Secondary legends are still a new thing for GMK, so those are still a novelty to warrant a full set, or in this case it could just be an alpha kit for existing Classic Beige owners + color packs.
Edited to add: that was just thoughts on the high amount of sets available and becoming available and how it might end up being in the [near] future. I still think this particular set in the IC has place as a full kit.