My son is saying the same thing - that they are waiting until she can’t be replaced - what day will that be??
Conceivably, a presidential candidate can be replaced right up to the day before the election.
There is no ballot restriction. Voters are actually voting for the candidate's electors, not the candidate.
The party would have to select a new candidate and make a public announcement to that effect, while instructing the electors to vote for the new candidate when the Electoral College meets (on the Monday after the second Wednesday in December).
They obviously waited for the early voting to begin. The Democrats strategy of pushing as many early voters through as possible will work against them now.