Satellite monitoring of 121.5MHz never worked anyway, due to false alarms and "accuracy" of several hundred nm.
Satellite monitoring of 243MHz stopped in 2009.
So a 406MHz watch is the way to go
I think the power requirement may be significant though... you can emit milliwatts on 121.5 and it will still get picked up on VDF equipment from say 10 miles away, which is all that 121.5 beacons were used for anyway for many years. But I think 406 needs a certain power to be picked up by the satellite.
However, Breitling would be perfectly happy to bring out a watch that is 100mm diameter, 50mm thick, and weights 1kg. Think of the extra pose value
After all, their current products are halfway there already, size-wise