I'm sure the Mormons wouldn't like it once the population of California outnumbers them as it inevitably will. This California is so utterly massive that I wouldn't be surprised if it gets carved up later on down the line. Although since it wasn't until the 20th century the interior had any decent population
I agree that the California part will eventually come to massively outnumber the rest, but it isn't true that the interior wasn't populated during the 19th.
In 1860, CA had population 380k, Utah had population 40k and the Mormon belt was more than that. In 1900, OTL CA had 1.5 million, UT had 275K, and the Mormon Belt had something like 400k-500k.
Frankly this proposal creates a real mess unless there is some sort of provision for automatic separation (maybe based on local plebiscite or something?) If so, I think you get a huge CA that includes much of OTL Nevada and AZ, and a Utah that is bigger than the present day one.