Donald Trump has done so much more than merely cast doubt on the legitimacy of the election's outcome.
He tried to overturn the outcome with a ridiculous strategy, steeped with conflicts of interest, that involved his Vice President refusing to include states they lost during the certification of the electoral college vote.
Among other things, this strategy included acts of election fraud. Some of these "alternate electors" were foolish enough, in states like Georgia, to sign their names to documents in which they falsely claimed to already be the lawfully chosen electors from their states. Some of them have already pleaded guilty, and they will go to jail. Deservedly so.
Your "what-about-ism" doesn't cover this alternate elector strategy, and Trump's attempt to overturn his own defeat.
Also .... the fact that Donald Trump has whined about election fraud in every election should be an indicator that regardless of the circumstances, Trump would never be man enough to admit that he lost a fair election. He's too much of a sniveling narcissist.
1) During the 2016 Republican Party primary, Trump accused the Ted Cruz Campaign of cheating to win the Iowa Caucuses. He never provided any proof in support of this claim.
2) During the 2016 Presidential General Election, Trump claimed that Hillary Clinton finished with lead in the popular vote because 3 million illegal immigrants had all voted .... and done so, for her. He never provided any proof of this claim.
3) During the 2020 Presidential General Election, he made countless provably false claims concerning fraud..
Can there be any doubt that he will allege fraud again next year? And his cult will go along for the ride .... one last time.