I guess I just don't see how it couldn't be just like that for Disocvery as well: Sonequa Martin-Green as the series regular; everyone else made up of Major Recurring Guest Stars.
I already explained that. You can't run a television series where only one person is under consistent contract and everyone else is "interchangeable".
With the original Star Trek, the series revolved around Kirk, Spock, and McCoy; they're the only characters who appeared in every single (or nearly every single) episode of the series, and are the characters through which nearly every single episode of the series was focused. The other characters occasionally got focus, which is how they became as beloved as they did, but the original series was, by and large, the "Kirk/Spock/McCoy" Show, and it didn't matter whether or not the other characters were present.
Let's take a look at a couple of hypothetical scenarios based on your idea of SMG being the only "Series Regular" and everybody else being "Major Recurring Guest Stars":
- We're introduced to, for example, the following characters in the Pilot: Burnham, Lorca, Stamets, Saru, Tilly, Culber, Tyler, Yeoh, Nambue, Connor, Anderson, T'Kuvma, Kol, L'Rell, and Sarek, and the show sets up both the Shenzou and Discovery as being intimately involved in a conflict with the Klingon Empire. However, in the next episode, only Isaacs, Jones, Serpico, and Frain are available. You'd be left with an episode in which you can't actually progress the story since the only characters you'd be able to have Burnham interact with would be Lorca, Saru, Anderson, and Sarek.
- You have the actors/characters listed above available for the first two episodes of the series because they're shot back-to-back, but when it comes time to shoot Episode 3, suddenly only SMG, Obi, and Chieffo are available, leaving Burnham as the only Starfleet character you can show, so unless you have her be captured by the Klingons and ignore wherever Episode 2 left off, you can't actually write anything because you don't have anybody to join her in crewing either the Discovery or the Shenzou.
Having Series Regulars allows you to actually operate a television series; it determines what ACTORS you have available to you on a week-to-week basis, which in turn determines the CHARACTERS you have available to you on a week-to-week basis, although it's not always as "cut-and-dry". Going back to Game of Thrones, here's a listing of every Series Regular for the first 7 seasons:
Season 1
Season 2
Season 3
Season 4