[_flux]

How much rechargeability really requires from hardware point of view?

I suppose the problem is that there are no standard for tiny magnetic chargers/cables. Every watch comes with their own, and they need be custom designed. For a device this large as much of the charging electronics should be outside the ring.

And another (small?) problem is that you'd need to electrically protect those external pins.

[noveltyaccount]

I'm with you, a little pogo-pin connector with all the charging circuitry in the external dongle would add, I don't know, ten dollars to the BOM. Very cool product but I won't buy a gadget I know I'd dispose in two years.