LBJ took the oath of office on AirForce one. They had no plausible way to "prevent Trump from taking office". In their fever dreams they might have been able to disrupt the parade, that was really their goal. Make a spectacle.
But, they mostly failed at that too.
Bob Avakian and Bill Ayers and their comrades have spent the last half-century trying to overthrow capitalism; what you or I might find plausible doesn't constrain their thinking. Beyond hoping to disrupt the parade, one of their broader goals is to make the situation "ungovernable", in their own words. Towards this end, they have also attacked Trump's legitimacy by stressing that he lost the popular vote, by painting him as a tool of Russia, by having John Lewis label his appointees as racists, etc., as meanwhile the press has been publishing misleadingly-captioned photos to "prove" that the anti-Trump protests drew more attendance than Trump's supporters at the inauguration. All of this is laying the groundwork to try to impeach Trump, which is one of the left's longer-term goals:
Confidential David Brock Memo: Defeat Trump Through Impeachment: Private memo lays out plan for next four years.
"Before election night, he had concocted a plan for what he would do if Trump won. . .By the time he woke up, he had the beginnings of an answer, which he tapped out in a memo to his fundraising partner Mary Pat Bonner. The basic idea was to double down and update his organizations for the next fight, with new technology, new targets and a singular focus to hobble the Trump Administration at every turn. . .: "Liberals Plot Revenge as Donald Trump Assumes the Presidency"