Which is often just not that necessary... must people's income levels in retirement are low enough that CA's income taxes are competitive with many other lower-tax states (not those with no income tax, though, of course).

That's just not true. You can fully move and start working for a non-CA company and you wouldn't be paying CA taxes.

You make it sound like if you ever lived in CA you have to pay CA tax for the rest of your life until you retire lol.