that's also possible on any other apple watch.
Yep...didn't say it wasn't
True, for many of the carriers, but I'd see greater adoption in Europe, as there's so much cross-carrier traffic there. And once the European carriers have it in place, that'll drive the others to do so.i enjoy your optimism, but series 3 watches will be long dead before this is implemented around the world.
Likely at least a few years away before you can count on it working everywhere.
But, with eSIM provisioning, it's quite likely that you could load additional carrier profiles on your Apple watch and switch to another carrier when travelling abroad. (you'd have to do a SIM swap in your phone if you want the number mirrored on the two devices). And, yes, the watches have limited LTE band support per SKU, but they have enough bands that they should work in most countries.