I find it incredibly easy to believe. Pratt basically performed at his poll numbers, in a city that is bluer than the county and in a county that is bluer than a very blue state. Bass is establishment Dem (not by her resume but by virtue of her incumbency) at a time of surging leftwing populism. The idea that either of the Democrat candidates wouldn’t have preferred to run against Pratt is preposterous. So the conspiracy to tank Pratt would have to be against the wishes of both candidates who benefitted from this conspiracy. Also, the forces that tanked Pratt were fine with letting the Trump backed Republican gubernatorial candidate make it to the general? Really? Also the demographics of Raman voters are perfectly consonant with the habits of young voters. Last, California’s incompetence at counting votes has yielded GOP wins in the past. Assuming that the same delays now “prove” anything other than consistently lethargic counting is a giant logical error.
Meanwhile, the fact that a lot of low information people think it was obviously stolen means nothing to me in terms of how to think about the facts, but you’re right that it matters politically and culturally. That people who should know better are letting their motivated reasoning get the better of them and are stoking this belief matters more.