I think it's a bit of both to maximise the cash grab. There are few franchises where people generally would enjoy all 10 films in a box set but I'd say this is possibly one of them (even if not to the same degree, ask anyone their thoughts on Insurrection...). Likelihood is they see an opportunity to release what they have for the mega fans, then drip feed the last of the TOS films, then uber box everyone with the full TOS & TNG film set. Maybe the reception to this set will determine the quality and production run of the rest?
I would imagine most Star Trek fans are fans of both TOS and TNG and would probably appreciate the whole set especially if it's not a barebones release, includes the TMP DC and so on.
I will certainly be opening my wallet and pouring out the contents for a 10 film 4K HDR set, if done properly. My feeling is this is the last physical format to come or to be to done with respect for collectors, I figure its the last time I will want to buy into these big box sets with the ever diminshing quality of included tat in most of them.
I would even be happy with two sets, one for the TOS film and another for the TNG films, but I've been stung at the wallet enough this year to not go double/triple dipping on releases with no promise of exactly what's around the corner. Especially given this isn't even all the TOS films; for me that makes this set an appetiser and not the main course.