Watches have a different cultural consideration, the good ones are meant to keep forever.
"But won't Apple lose money that way?" If they are upgradable, they'll sell more to begin with, and they'll sell the upgrades, and it keeps customers "sticky" with the ecosystem, it gets people using Apple Pay. Either way they make money.
The Watch is a whole new category. Gold, Stainless Steel versions will be expensive. It has to be able to have longevity.
There are Apple products that have upgradability - iMacs, MacBooks, Mac Mini's. Not full upgradable, but adding RAM or a SSD improves it a lot.
It's a highly personal, wearable. It's not meant to be replaced every two years.
The brains in the Watch is a sealed unit that can easily be made to pop out and pop a new one in.
Under the band attachment area, there's probably screws. Take those out, the thing comes apart. Pop new system-on-a-chip in and you are now using an up to date device.
There's no way they do this [only] with the Gold one, no way. If you think that is plausible, then you have to consider that the ability to do it has been engineered. Then what, make two versions of the underlying tech? No way. If the Gold does it, they all do it.
And Gold Edition will most certainly do it.