Apple is all in on Swift, so you will not be writing native MacOS or iOS code for UI in D, best case you put your business logic in D but you can do that in any language which has bindings to swift/Obj-C.
Android is all in on Kotlin/Java, not D again
Microsoft is all in on C#, again not D.
Linux your two best options for UI is GTK and Qt, C and C++ respectively.
So the only place where you could bave seemless integration is Linux through FFI.
Here's the thing though, for building a core layer that you can wrap a UI around, Rust has insanely good ergonomics, with very good third-party libraries to automatically generate safe bindings to a decent amount of languages, at least all those listed above and WASM for web.
None of those uses cases are painless in D.
It makes sense for a complex productivity app (e.g. an office suite editor) to implement the UI from scratch anyway, and for that they may choose D. If Jane Street didn't pick OCaml, it would've died long ago -- in the same manner, some company might pick D to do UI or anything else really.
Handling energy efficiency/a11y/i8n is non trivial in any language, using the paved road of the system's native implementation solves for many of those problems out of the box.
You would need to reimplement all of that in D lang for your UI layer, and all you wanted to do was build an application to solve a problem, you weren't in the business of building a UI library to begin with.
You're free to start a company and solve a real business problem.
If I became a D expert tomorrow it's not going to make me more money. It's not getting me a better job.