Sam, you’re making a “both-sides” argument here: that both sides are increasingly radical, moving away from the centre, and the solution is for both parties to dump their fringe elements and move back to the centre.
But that’s not supported by political science research into the US party system, and comparing it to parties in other countries. That research shows that the Republican party has been on a steady march away from the centre and to more extremist positions, while the Democratic party is still in roughly the same spot as it was 40 years ago.
From an Economist article at the last presidential election:
And the article has a the chart showing that march to extremism by the Republicans It shows that the Republicans are past the UKIP and National Front (France) and are getting close to AfD (Germany) and Fidesz (Hungary)
The Dems, meanwhile, have moved up a bit on the populism axis, but are still bopping around roughly the same spot as the were in the early 80s.
And a similar article and chart in the NYT:
Maybe this is an example of the Overton window: as the Republicans move to an extreme, that may make the centre move towards that extreme, even if the Dems haven’t actually moved much. But that’s not the case of the Dems becoming more extreme: it’s that the more extreme Republicans become, the greater they (correctly) perceive the difference is between them and the Democrats.