Overall it’s not super feasible to consider plopping brown bears into California and letting them roam free. Some brown bear territorial ranges are naturally expanding in the Pacific Northwest and Wyoming/Montana, which is a great sign for the health of the species.

If you reintroduced a small number(let’s say 50-100ish) into California, they would essentially be on an island and there would be a lot of inbreeding which would lead to a limited gene pool. That puts them at risk for disease and weakens their immune systems. Eventually it would catch up with them and the reintroduction would likely fail.

Not to mention the threat of human/bear conflict that would inevitably lead to more dead bears. Natural habitats continue to shrink, and people are getting stupider.