[orange dude, angry, fist raised] I will protest against both sides of the evil capitalist elites that are ruining society by voting for an anti-system extreme!
[pink gal, looking candid] Since your protest is based on human values and a desire for more social justice, this means you’re going to vote far left?
[pink gal, looking scared] Isn’t that right?…
[orange dude starts laughing so hard tears comes from his eyes and his hairpiece starts flying]
Even if all the far-left parties only get 1% or 2%, if the moderate left arty doesn’t have a clean majority, they need the votes of those far left parties to form a government, and thus the far left parties can negotiate some stuff. It’s not useless to vote third party.
Except in the US the moderate left generally chooses to cater to the right when they need to bolster their numbers, rather than the far left. It makes sense given that our current Democratic party is far closer to the moderate right than the far left. We’d need to elect new incoming lefter-leaning democratic leaders across all levels of government over the course of decades to get the party as a whole to tilt leftward rather than rightward when a clean majority is needed. It’s still the most viable option, but it’ll be a lot of work to achieve.