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.

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.