The big question here is if journal space is a limited resource. Obviously it was at one point.
Supposing it is, you have to trade off publishing these incremental results against publishing someone else’s complete result.
What if it had taken ten papers to get there instead of two? For a sufficiently important problem, sure, but the interesting question is at a problem that’s interesting enough to publish complete but barely.
The limiting factor isn’t journal space, but attention among the audience. (In theory) the journals publishing restrictions help to filter and condense information so the audience is maximally informed given that they will only read a fixed amount
Journal space is not a limited resource. Premium journal space is.
That's because every researcher has a hierarchy of journals that they monitor. Prestigious journals are read by many researchers. So you're essentially competing for access to the limited attention of many researchers.
Conversely, publishing in a premium journal has more value than a regular journal. And the big scientific publishers are therefore in competition to make sure that they own the premium journals. Which they have multiple tricks to ensure.
Interestingly, their tricks only really work in science. That's because in the humanities, it is harder to establish objective opinions about quality. By contrast everyone can agree in science that Nature generally has the best papers. So attempting to raise the price on a prestigious science journal, works. Attempting to raise the price on a prestigious humanities journal, results in its circulation going down. Which makes it less prestigious.
Space isn't a limited resource, but prestige points are deliberatly limited, as a proxy for the publications' competition for attention. We can appreciate the irony, while considering the outcome reasonable - after all, the results weren't kept out of the literature. They just got published with a label that more or less puts them lower in the search ranking for the next mathematician who looks up the topic.
Supposing it is, you have to trade off publishing these incremental results against publishing someone else’s complete result.
What if it had taken ten papers to get there instead of two? For a sufficiently important problem, sure, but the interesting question is at a problem that’s interesting enough to publish complete but barely.