The appeal of swift is that it is lightyears better than ObjectiveC. To ObjC developers that are used to being treated like trash by Apple (abysmal documentation, stone age IDE and tooling etc) that's huge
Apple's documentation was actually vastly superior before Swift. In fact, I often refer to the documentation "archive" instead of the latest docs.
The decline in the docs is due to a number of factors, but I would say mainly it's (1) the relentless annual major OS update schedule, (2) the proliferation of OS (macOS, iOS, watchOS, tvOS, xrOS?), (3) the dual language stack, (4) Apple personnel turnover.
Apple seems to abuse the fact that people make a living translating the lack of documentation into online courses, books, and articles. The market for this is an ongoing enabler at this point.
I do quite like Swift overall -- it’s usually terser than Obj-C and I like the much stricter and more expressive type system. But you pay for that with much longer compile times, and the tooling feels much the same (Xcode is still Xcode).
