Good one overall, although I’m not sure I can agree that “good ideas tend to win elections” as much as we might like. Trump brought a lot of bad ideas the his campaign that a substantial majority of people disagreed with, but ended up winning mainly because of the weak candidacy of his opponent. Personality and popularity has always mattered when it comes to elections, and perhaps that’s even truer now.
I do agree with your opening thought. Winning parties have no incentive to change their approach based on high third-party vote counts, because for them, the winning has validated everything they campaigned on. Same as when a candidate wins an election by the thinnest of margins, then claims a broad mandate to do everything they ever wanted for themself and their benefactors that is antithetical to the losing party’s voters. Just by logic, we can see that how many votes the losers end up with matters not a whit, so the whole idea of sending a message with a third party vote crumbles. Winners win and just do what they want anyway, because they believe they have earned that right.
So when it comes down to it, voting for a third party candidate you already know is going to lose is an exercise in futility and, depending on how much you want to brag about your political savvy for voting third party in the first place, vainglory. I’s your right to do so at any time, and no one can prevent you from doing so. It’s just that there are times when the effect of your protest vote on the entire body politic, and body social, matters more than others, which should inform a voter’s decision on voting third party versus holding your nose and voting for the lesser of two evils.