If you want to develop software for the Apple ecosystem, then yes; otherwise, it’s probably a waste of time.

If you're 100% in on Apple, it's probably fine. Outside that ecosystem, Kotlin, Go or Rust are all probably better investments of learning time depending on the problem you're tackling.

The question more is, is bothering to learn native smartphone app development a bad investment of time now that the low hanging fruit is gone and we're due for a new platform paradigm?

As an iOS dev, IMO I’d say it’s useful if and only if you’re writing native apps for Apple hardware.

While Swift can be used for linux desktop or as a backend language, what mostly matters for practical projects is the libraries, and I think for those cases the better libraries are on other languages.

Are you writing Mac or iOS apps? Then, no. You'll pretty much need to learn Swift. Objective-C is clearly in legacy mode. You can't even use several APIs with Obj-C.

Are you writing something else? Then, it's probably not the best investment to learn Swift.

Others have covered why/why not.

I propose learning Kotlin if you want something syntactically similar but with broader set of usecases or Rust if the lack of runtime with modern syntax is what is appealing to you.