This is exactly why you need a functioning multi-party system to breed steady progress and trust in political institutions. With a multi party system you will have fringe parties getting a couple percent of the vote.

For example in Norway, the party "Red", which is literally a communist party with armed revolution as part of its official party programme, regularly gets like 3% of the vote, and more-or-less nazi party that get <1%. Then you have a bit more moderate parties that get maybe 10%, before you get to the classic "labour" and "conservative" parties, which gets 20-30% each, and a couple "center" parties with ≈5-8%.

The point is that when one of the extreme (or centrist) parties suddenly starts growing, the big parties are forced to change their policies to regain votes. This also means that a typical government is a coalition between several parties, where the distribution of power between those parties is representative of the vote. For example, one election the not-so-extreme left party could get 12%, forcing the larger Labour Party to tend left on a lot of issues to gather enough support.

In short: A functioning multi-party system with coalitions favours nuance and gradual political change, rather than a black-and-white, polarised system. At the same time, everyone can see that their views are actually represented in the parliament, because even if they have extreme views, they have a party to represent them. Over time, this likely makes people less likely to grow even more extreme, but encourages them to vote and work within the political system, while recognising that their views only hold 1-5% support in the population.