The developers that use XCode with Objective-C probably won't feel any difference as XCode will likely just silently compile the code into ARM (or both a la universal package from PPC days). It's too early to tell though.
there are a lot of programmers that use mac OS that don't even touch Swift or Objective-C. I'm in the web development industry, and I'd say 50% of the developers I know use apple computers for their job. Java, Ruby, even C#.
that seems a little simplistic. It isn't just the runtimes, but also all the tooling that goes with it. There would be a lot of work involved with recompiling all tooling and runtimes to work on ARM, and would probably alienate a large portion of developers currently using Apple products as a result.
This was already done twice in the past, once on transition from Motorola 68000 to PowerPC, and once on transition from PowerPC to Intel. All arrived with updated versions of interpreted language runtimes. We'll have to see what will they do this time.
Only if you have a dedicated gpu. Because OSX can easily switch between integrated and discrete.
On my Macbook 12" I get 2 extra hours out of Windows 10. But on my old Retina Mac Pro I get almost half the time out of Windows 10.
It's a shame that Apple wont go AMD because the battery life would be awesome on OSX with discrete gpu.