This was always the plan. They will contest any replacement and then only Trump will be on the ballot. AOC tried to warn you fucking people. This is why Biden needed to go over a year ago and the PARTY said “fuck you guys” until it was too hard to fucking ignore.
It’s not a federal legal matter, party internal politics are not regulated the same way as actual elections. There’s some tangential finance laws, but the democratic party can pick whoever the hell they want however the hell they want.