it's the modular design that's a new thing (the flanged adapter piece.) Traditional seacocks have been around for quite a while and they also have the flanged base and are perfectly fine. A friend has them on his 30+ year old Whitby. The down side to the traditional seacock is that if the valve fails the whole thing has to be removed in order to replace it although I'm sure that's a rarity. Your boat could have them. If so, I'm told they're quite rebuildable.

Groco's flanged adapter design allows the valve to be removed without disturbing the thru-hull and the adapter.

You're right though, most production boat builders just thread the valve directly onto the mushroom but I suspect it has more to do with the bottom line than anything else.

I now have a better understanding of why well made boats are so darned expensive.