That may be true in Spain, where iirc it was split 4 ways, correct? And 2 of the growing parties were populist yet diametrically opposed?
But look to the rest of Europe and see what happens. Sweden, Ireland, Netherlands, Germany, and so on. They have a populist party. The math says it makes sense for them to get into government on a coalition and get 2 or 3 of their policies implemented (which are overwhelmingly popular yet the mainstream doesn't touch) and what happens instead? The central parties close ranks and form incongruent alliances to deny the new party legitimacy.
This type of behaviour isn't unique. Trump is a good example of even when the populist gets power, he's subject to getting hamstrung by his alleged 'allies' behind closed doors. But the fact remains a party can get 20-30% of the vote share in a multiparty system, and have the momentum on their side, and the other parties are under 0 obligation to go into government with them if they don't want to.
In the likes of France or the UK, the reply seems to be neoliberals consciously embracing the trappings of what makes these populist movements popular, and embracing one or two surface-level reforms, but at the end of the day it be a rebranding of the same types of people that have been in power before the 2008 Recession. It's fascinating how everybody is trying to get out in front of and exploit the blatant disillusionment so many have with contemporary politics, but doesn't distract from the reality that established politicians and powers that be aren't gonna just wing it and give the new faction a chance at power unless their mandate is undeniable.
To loop this back to Joe, the old order would sooner install a senile old man by any means necessary than have Trump upset the apple cart any more than he's already done. Democrats want him gone despite consciously having no message for their base to be passionate about, and traditional republicans because he's trying to fix the party without their blessing.