If you name your product Champagne, in my mind it's totally fine for someone to start making Californian champagne.
These are products which are literally defined by their geographic origin.
Champagne is a sparkling wine from the Champagne region, that’s what it means.
Your logic means that there is nothing wrong with a “Californian Champagne” made in Texas.
The issue isn't so much that there are / are not geographic monopolies, but that you have 2 different IP/economic/governance models expecting to be speaking the same language -
Appellations are connected to producers associations which ultimately have their roots in feudalism/guilds/etc. Generally, it's not the region that promotes an appellation, it's the producers association of that product/region.
Producers associations are essentially a "cartel" in modern economic terms which would be illegal under modern IP/economic regulatory regimes. Where appellations are used, this is not the case, and the associations are allowed certain legal space, some of which include creating appellations, which to some extent provide a way to 'brand' and so add value to otherwise commodity products.
So, to allow for appellations, you create some provision in the legal system to recognize them. However, since allowing appellations ultimately means granting some political legitimacy to organically occurring cartel-like organizations rooted in feudalism, this can be problematic if using a post-feudal legal framework. The US comes down more radically anti-feudal and pro capitalist in it's legal code than other western countries, and so you have more resistance to appellations.
In this model, 'champagne' itself is the brand and the trademark. Nothing else is champagne. Unfortunately, no specific entity 'owns' this brand, because the 'brand' was developed according to an informal patchwork of local organizations, customs and traditions, and so you have some systems (e.g. USA) denying the legitimacy of the claims to the 'brand'/'trademark' since it isn't 'owned' by 'someone'.
Something made elsewhere can be similar? Yes it can, it's a whine. Will it be the same? I doubt it.
The region is known to produced it since roman times, adopted by french royalty for more than 1000 years... but because it's a region shouldn't be entitled to it?
If Champagne is a whine, why not call it Californian Foamy White Whine?
Even better, stop referring to it exclusively by the location, as if Champagne, France is not capable of producing other wines. Then you can have "Champagne sparkling white wine" and "California sparkling white wine" (or even "Champagne-style California sparkling white wine"). Of course, colloquially people will still refer to it as champagne, but I'm talking about what's printed on the bottle / marketed.
Well some cultures have different words specifically for that.
>Even better, stop referring to it exclusively by the location, as if Champagne, France is not capable of producing other wines.
I don't understand this part, because I see no where implicit that Champagne doesn't have the capacity of producing other wines, just by calling Champagne to Champagne.
For example, where I'm from - Portugal - we make the distinction between Champagne and "sparkling wine" (I couldn't find a better translation, the word is Espumante) - even if the outcome is an attempt to make it similar to Champagne, it's not champagne. It's two different words that define different things.
You go to a restaurant you ask for Champagne, you get Champagne. You ask for Sparkling Wine, you get Sparkling Whine.
Because when you buy Champagne you want wine with particular properties that are particular from the region. If you don't like Champagne, you don't want wine with those qualities and you ask for something else that is not Champagne, but yet can still be a white wine with foam.
Plus you talk about this like this is a gimmick.
It's not something "new", it's been like this for centuries, it's in our dictionary. Not mandated by law, or trademarks, - it wasn't a marketer that woke up one day and decided to make that "growth hack" - it's cultural.
To us it's like calling two distinct fishes the same name just because they are blueish and have spots.
Same for example, to Porto. Porto Wine is done Porto region, has a particular taste. You have other liquors, but Porto is Porto.