What will happen with Mac as a dev platform after ARM?

We'll find out more later on today, I'd imagine, but my feeling is that ARM on desktops is likely to be ~2 years away. If you buy computers about every 2 years, go for it. If you buy computers every 5+ years and can wait, then wait.

There will be a rough period as toolchains are ported over and then it will be business as usual.

What kind of dev?

Web dev or embedded dev? Shouldn't make any difference at all whether you go with Intel Mac, ARM Mac, Windows, or Linux, except for your personal preferences regarding which OS, tools, and (maybe) the platform you will deploy to. So yes, it's still worth it today iff it was worth it (to you) last year.

Native Mac or iOS app dev? Yes, it's not only worth it, but it will be your only option for a while and then an equally good option for a few years thereafter.

Most toolchains are cross-platform already. Even some that build to native code. Systems programmers may steer clear for a while but they probably already weren't using Macs.

Have they demoed anything with bootcamp? I imagine that anyone doing Windows work could be impacted?

Why would anything change?