You seem to be forgetting the 3.5 year long wall of propaganda we were inundated with claiming that Donald Trump was a Russian puppet and Russia hacked our election. You had prominent, important elected officials accusing those that disagree with that assessment of "carrying water for Putin", " doing the bidding of a dictator", things like that. You had many of the same people calling for unrest in the streets until Donald Trump is removed from office.
If that isn't a sustained effort to paint the 2016 election as illegitimate I don't know what is.
I'm not saying the election of 2016 was legitimate. I never claimed the investigations and claims were bogus, just that they occurred. I'm responding to a claim that this election is called illegitimate by partisans whereas the previous one was not. Whether either election was legitimate or illegitimate, political opponents of the victors put in great effort to paint them as illegitimate. Both sides are playing this game, and to demonize the one doing it now while forgetting that it has been happening for 4 years already is disingenuous.
If that isn't a sustained effort to paint the 2016 election as illegitimate I don't know what is.