"we need to have an open convention to give the people a voice for choosing the nominee"

 "The people" for the most part are not invited to the Democratic National Convention.

Meanwhile, "the people" elected Harris as Vice President. The central role of that office is to be ready to substitute for Joe Biden if he isn't up to it on his own.

There's a better argument that Harris has a popular mandate to be the nominee than anyone who the delegates at the convention may choose.