You line of thinking is solid. I predict that within five years, all of this technology will be on our phones and the primary question we will have will be in regards to which earbuds are best for noise isolation, sound quality, and allowing in some of the ambient sounds outside the helmet for safety.
The helmet bluetooth system is going to go the way of the GPS. Some people will continue to pay high prices for them because of specific needs, but everyday riders will transition to a phone app that allows for the similar functionality and provides greater flexibility for the rider. If I'm an executive at Cardo or Sena, I'm putting aside 10% of my R&D budget to focus on a quality app and then teaming with Bose, Sennheiser, Jabra, or some other headphone company to produce a bluetooth headset (the kind that sit on the neck to allow for more battery life) that works in conjunction with helmets. Finally, when the battery technology catches up, transition to earbuds speakers.
I have to say that I'm very surprised and disappointed with my findings. I bought an iPod late in the game, and that was about 11 years ago, that's about when BT came out and yet I buy a set now and they aren't even stereo and last 1 hour at full volume. Sad, I guess it's because people want the cheapest thing they can get.
I bet those you bought really sounded good. I saw ones with a driver about that size and thought they'd have trouble in the on/off process.
I just tried out my original Apple wired earbuds and they make a HUGE difference based on where they are in the ear. Once they get lose, the quality drops like a rock. I think if you try the smaller driver type, you'll have better luck, it's just a matter of finding ones that are stereo, sound good and have some good battery life.
One note on the neck band type. You're right about the battery size. What I did was took a 3.7V phone battery and hooked it up to my headphone amp. I have a battery many times what is actually needed. This should be done on the helmet, a standard 18650 on the helmet should last all day or even several days and you won't have to mess with the neck band.
Back to the speaker option. I was looking for a rubber bladder to control the wind noise and came up with the idea of getting one of those thumb pumps and a rubber bladder to make the areas in front of the ear nice and tight.
Some expensive helmets have this built in, mine has adjustable foam, but I have to pull it out and it make it damn tight to fit and you still have wind noise.
The more I think about this, the more I want to do a Kickstarter to design a more modern helmet that has all these features. Built in sun glasses, dual lens for fog, better air flow, built in battery, speakers and amp...