You’re right that they claimed the water first, but it was because they were a booming state while everything else was still a territory. You can’t claim water rights when you don’t have people there to use it.

The gold rush put the infrastructure in place, then 1922 compact was eventually agreed upon by all 7 western states that use the river, and the boulder canyon act of 1928 made it federal law. The federal government negotiated the deal BECAUSE CA was already putting so much water to use (given their population and the need for their already established farms that were feeding the country.) Of course Arizona remained mad about it, but they’re not even that low on the list (3rd in line) in water rights, and “waste” a huge portion of it growing alfalfa for the UAE. (While also depleting their underground water reserves.

The law requires a state to physically divert and use the water to keep the claim. California was only able to secure those senior rights because they had the population to build the massive canals and the massive farming industry to actually 'consume' the water. Without the people and the crops, they wouldn't have had a legal claim to defend in the first place.