I don‘t think people believe in the effectiveness of the proposed solutions.
It’s more about “the old parties didn’t have any solutions” and “political promises and proposed solutions don’t work anyways”. So why not try a new party
> political promises and proposed solutions don’t work anyways
This is very effective for populist parties because in a way it keeps them from being held accountable by their own voters. When I hear interviews with Trump voters being faced with evidence of how Trump hasn't kept one campaign promise or another, it's exactly the sentiment above: politicians never keep their promises anyways and Trump is no different but we like him so we'll stick with him.
I don't know about german politics, but here in the US this isn't what I observe. My aunt is a republican lobbyist. She is also a drunk. She gets drunk and texts us her unfiltered thoughts.
She genuinely believes that the bulk of racial minorities in the US should be expelled from the country, that Trump is making progress towards that goal, and that the US would be a better place if it was just white people. She isn't just motivated by frustration with the liberals. She is motivated by the racial purity policies of the GOP.
