I don't think the fear in most politics is about the actions of the 'newcomers', but about potential of the old guard losing power. In the US if a third party started looking like a real threat, Rs and Ds would 100% come together to frame them as the greatest threat of humanity to ever exist, exactly as happened with Perot even in the age before hyper hyperbole.

Like you alluded to Meloni went from end of the world to business as usual in remarkably short order, and the same could viably if not likely happen with AfD as well, but the bigger change behind the scenes there is that her party went from 2% in 2013, to leading Italy today - and that gain came at the expense of the loss of power by whatever the older establishment was in Italy.