I’m wondering if this maybe kind of thing was always on the roadmap.
Apple has had a number of things where they start out with a more restricted version - like, say, the one-button mouse - and then once support for that is firmly established, then they open the access a bit more. In the case of the mouse (which is likely a weak example - Steve probably really did want just one button - but its easy example to grasp), requiring all the software to work with one button, established a pattern, then the addition of multiple-button mice allowed additional tricks, but didn’t require everyone to switch to multi-button mice, because there was a huge amount of support for single-button mice already written.
Similarly, if TV+ had launched with a few dozen recognizable existing TV shows and their originals, the tone and expectations would be different, and Apple’s original shows would have gotten lost in the noise. People would be debating whether “Friends” (or whatever show) was a worthy reason to subscribe, instead of being focused on the merits of “The Morning Show”, “See”, or my favorite, “Mythic Quest”. So, launch with just the originals, but have personnel actively considering what shows they might want to buy, and researching what shows they can get.
I suspect they’ll acquire additional things at a slow rate, if they feel the show in question has something to add, and they can actually get it (many shows are being consolidated under their own company’s streaming services). Unless they go nuts and decide to buy an entire service. Unlikely, but possible.