By choosing the less shit option, politicians will see they need to be less shit to get elected, eventually to the point of maybe even having good candidates.

This is demonstrably false. After Obama was elected twice we got Trump. Clinton was picked in 2016 and was so terrible that she couldn't beat the orange turd. After her loss they gave us a "status quo" clone of her who barely managed to defeat the orange turd. Now we're faced with the exact same choice and polling shows that it's likely to end up like it did in 2016. Both parties continue to move further and further right regardless of who's getting elected and we're being forced to choose from the same tiny pool of candidates every election even though there are hundreds of millions of people in this country.

Continuing to vote against the other candidate rather than voting for who you want allows them to keep doing worse and worse because that other party is always going to be there and always considered worse and supporters of that party will look at the other side and do the same. There is zero accountability for either side to the point where both candidates now openly support genocide and you have people arguing that "you're a piece of shit for not supporting them."

By continuing to vocally support eating shit, you're ensuring that in a few elections we'll be supporting eating shit and lighting ourselves on fire because the other side will be eating shit, lighting ourselves on fire, and giving a rim job to a horse. To support our current system is to support a race to the bottom.