The NEW extension doesn't requires to restart the platform, while UPDATE requires.
I found the following thread which was posted years ago, is there any improvement for the behavior now ?
Not one I am aware of.
This, and extension development generally, is less then ideal for me as well, due to an additional unrelated reason: debugging extensions, which implies adding some JVM parameters, opening a port...
For this reason, when I'm doing heavy extension development I'm doing it only on my local or directly accessible server through SSH. When finishing, only one restart, and things are done...
I suggest to raise this as a Product Idea if it really impacts you.