2020 showed that a lot of democratic senate candidates could have benefited from a tough primary- I largely think this is a good thing!
I find this claim intriguing, since every Dem who won a competitive seat in 2020 had cleared the field months beforehand (Hickenlooper, Kelly, Ossoff, Warnock) while the vigorous primaries seats were all lost (Cunningham vs Smith, Greenfield vs Franken, Hegar vs West)
Of 8 competitive seats, only 1 really makes your case (Maine) and Collins' personal brand there make it difficult to infer any course correction for the future.