Except that assumes that Johnson's officers and men follow him in trying to seize an area which voted overwhelmingly against secession, which is not certain.*
That pro-Union forces, including the navy and other army units in Oregon and various territories don't go along, not to mention the pro-Union majority.
Lastly that the alleged pro-secessionist forces actually prove capable of doing anything, bearing in mind that throughout the ACW pro-secession/CSA forces repeatedly turned out to be worthless when it came to actual action. The classic being the Midwest in 1864 where organizations with 200,000(!) members were asked if 500 would actually be willing to take action, for just a few days. Nope.
In this case all Johnson has to do is violate his oath, convince his officers and men to do the same, seize the arsenals and other major military bases while dispersing the elected state government and any opposition forces in which case the pro-CSA forces may join the winning side to some degree.
Far more likely in this event is that the Union retakes whatever the CSA took west of Texas in 1862 as the CSA simply doesn't have a viable supply line in this area while the image losing such a large area might be a problem for the CSA, not to mention trying to seize an area where 90% of the voters went against secession candidates in 1860. Going for southern California, of course, effectively endorses West Virginia and invites further efforts in eastern Tennessee and western North Carolina among others.
*As for Johnson's morals, they were not so readily apparent in his handling of the so-called Mormon War against Utah and there is some evidence that he may have been forced to abandon his stated intentions towards the Mormons under duress from his own subordinates. All history knows for certain is that:
A) Johnson announced his intentions to occupy Salt Lake City as his capital and place the territory under martial law.
B) Brigham Young, who had thus far avoided direct military confrontation, vowed that his patience was also about to end.
C) By all accounts eyewitnesses thought this was indeed about to explode into a full-blown shooting war.
D) The next day Johnson led his troops into Salt Lake City and then led them right out again without the slightest warning that he had changed his mind about a military occupation and giving an impression of a man who was very angry about the actions he was taking that day.
E) The senior officers in Johnson's force, all northern or Union-sympathizers, itself unusual given the percentage of southern officers, were inveterate diarists and yet strangely none ever felt the need to write about that night and why what he announced publicly one day, in front of journalists and a representative from President Buchanan, was the opposite of his actual actions on the next.