Your parents, assuming they are old enough to be seniors, will be fine. Your well-founded concern has been heard and handled. You can rest easy, knowing that your parents and other homeowning elders in California will not be forced out of their homes by property taxes they cannot afford.
Nobody likes paying taxes. It's literally never a pleasurable or enjoyable experience. Yet it's one that furthers society and allows it to do things like fund schools - a chronic problem in California.
Not disagreeing with your points, but one major reason that it's "never a pleasurable experience" to pay tax in America is that ideologues deliberately designed the process to be as painful as possible, to give taxation a bad name. Sadly, it's working.
> I set out on this ground, which I suppose to be self evident, "that the earth belongs in usufruct to the living": that the dead have neither powers nor rights over it.
(Though really, even before I knew of Jefferson's attitudes about inherited wealth, I'd read enough to know that divesting children of unearned windfalls isn't necessarily a bad thing. Can't take it with you, ya know.)
If parents bought a home in CA in 1980 for $300k, and then did little more than regular upkeep and minor improvements, and then die in 2021, why should their offspring be entitled to a $10M windfall? They've done nothing to earn it, besides being lucky to be born to parents who were lucky enough to be able to buy housing in an area that they were lucky enough to keep living in when that area became popular.