In the US third parties are more or less legally prevented from winning due to all sorts of legal codification of the parties in state election laws. For an election to take place in Ohio (as an example) without Dems or Reps existing would necessitate rewritten election policy. ”Anti-corruption measures” necessitate a registered democrat and a registered republican are together with the ballots at all times. So it’s not this simple by any stretch of the imagination. Dems and reps have legal power other organizations do not. They use this (beyond the simple inertia they maintain) to ensure they hold power. I cant speak on parliamentary systems so if it was intended to apply to that, fair enough. Voting third party in the US (at best) indicates to the closest ideologically aligned major party which way they need to move their policy to capture the fringes.

EDIT: another comment I read mentioned that a third party reaching a certain percentage would open funding and while I’m not positive on how that all works makes sense to me. So while I’m probably being a bit overly dismissive of third party voting I think my point still stands for the most part.

PS this reality evidences the necessity of revolution not continuing to ‘vote blue no matter who’