Not all Mac users are devs.
Its not that all users are devs. Its that all devs might not be able to make their software work well under that environment.
That's for servers and scientific software (and perhaps 3D and such).
For regular devs the massive core count helps even with non optimized apps, because unlike the above use cases, we run lots of apps at the same time (and each can have its core).
I think you will see a lot of performance boost after switching to ARM. If they start on the "low end" then a macbook will be practically on par with a mbp. This might not be useful at first for native development, but I am quite sure that macOS, iOS and web development will be very much possible on these machines - the three domains that Apple cares most about.
A battery lifetime of 8 or 12 hours is plenty, and going beyond that isn't that much of a marketable strategy, unless it has to become 24h+ or something. A lower weight approach however would also mean a lower BOM for Apple, and more profit, while being able to shout "1/3rd lighter!" - and that's an easy sell :)