Hillary Clinton has said that Biden should never concede the election no matter what:
Has Biden ever denounced that approach?
Democrats have spent the last four years claiming that the 2016 election was not legitimate. Antifa and its allies burned cars and engaged in the violence at the inauguration, and there have been continual violent protests since then. Clearly Biden did little to prevent violence during and after the last transition.
If Biden said there would be a peaceful transition now, would it even matter? Would the violent anti-Trump extremists care what Biden has to say?