It's like having a monopoly and not having to compete.

Really you want both sides to be strong and for there to be a healthy push and pull. So the best ideas from each rise to the top and then do battle via good faith debate. With the ability and willingness to steelman each other's arguments.

Instead what we tend to see is an eagerness to interpret everything in the worst way possible, attacking strawmen and talking past each other. Which accomplishes little aside from increasing polarization.

Not really. The farm valleys are very, very red, while the major cities are very, very blue. I would say CA is a very politically polarized state.

This is why I really wish we had ranked choice voting (or approval voting). That way we'd have more political parties and a healthier political climate.

California allows two candidates from the same party to run against eachother. Do other states not?

This is very wrong. Drive the 5 sometime. Outside of LA/SF it gets very red, very fast. Even within those areas it's only the centers that are truly "blue"; (AFAIK) Orange County is particularly conservative, for example, and it's always been my impression that places like Marin, Palo Alto and Orinda are the same way. It's where the term "California Republican" came from.

Look how long it took to legalize gay marriage and weed!

It's not even that it's one party. Since there's only two parties, there's a lot of variance within each, and you can be a Democrat without trying to make freelance work impossible, petty crime routine, streets smeared with human excrement and a trip in public transportation both rare and dangerous. I don't see why even somebody who would never vote Republican still can't choose not to vote for somebody like politicians who pushed through the idiotic AB5, for example. Or the ones who have been in charge for so many other ills plaguing modern California. I don't think it's party-against-party thing. If CA voters wanted, they could vote for much saner policies still well within the mainstream of Democrats (or at least something that has been mainstream of Democrats before 2016), but for some reason they do not.

California is a single party state in name only. There are plenty of conservatives who call themselves democrats in all levels of government. Many city councilmen in LA lean very conservatively in everything but the common sense stuff like not being a bigot, so they fly under the radar and people see the D by the name and vote them back in without reading a word of policy.