In my humble opinion, people who are bisexual don’t have a monopoly on the prefix “bi.” The terms “bigender” and “bipolar” also exist, which can be used by people who are not bisexual.
I can see the issue of people using the same term to mean different things - appropriation, misrepresentation, etc. But “bipoly” and “bisexual” are different words. In what way is the bisexual community harmed by other types of identities using the “bi” prefix?
It’s important to remember that identity labels, like all words, were made up by someone at some point. And there will always be people who have opinions about the process used to develop that word. But people who need words to describe themselves and their experiences will find a way, and as long as no one is getting hurt - if a term isn’t being stolen or appropriated, or slurs aren’t being used - I think it’s best to let linguistic change be part of a healthy process of growth and discovery, rather than a way to draw lines and challenge each other.
But, of course, you’re allowed to disagree. I don’t get to tell anyone what is or isn’t allowed to make them uncomfortable. All I can do is have my opinion and express it with as much honesty and gentleness as I can, and try and hold space for other people’s opinions.