It is a threat to democracy because, after 3 years, we now know that it is a power wrestling within the ruling class, for the exclusive benefit of the ruling class. All the good reasons you may have had before and right after the referendum are irrelevant now. What Brexit is and what it is used to do is clear for all to see, and it is not a democratic excercise.
To take a less loaded example: that's like sending money to a charity "Food for Africa Children" and realising 3 years after, your money is instead funnelled into buying a super yatcht for some billionaire somewhere.
Feeding Africa Children is good, giving money to charity is good but you can no longer make the argument that "Food for Africa Children" is good, and you can no longer claim any moral high ground if you continue to support that organisation.