There are certain issues here are legit and some that are not.
Campaign finance—not legit. The current bank balance of the Biden campaign is less than 10% of what is needed for a national presidential campaign. At worst a new nominee suffers a 10% penalty—no biggie.
Filing deadlines—don’t buy it for a minute. The convention dates have been known for a year. The courts of the states will require the nominee of the convention to be allowed on the ballot. It may require some litigation—but imho the end result is not in doubt.
The legitimate issues are more political in nature—separate and much trickier topic.
These are bigger issues than you make of them.
A candidate cannot merely hand over campaign contributions made to his or her campaign to a replacement, and why would they even want to? But you are also asking huge donors to accept that the replacement will represent their interests the same even if they’ve given as much as they intended to or even can. At the end of the day, everyone is in it for themselves. And money will greatly exacerbate this.
The convention dates are not an issue either. Conventions are supposed to revolve around the realities of the state laws, not vice versa. While Democrat party rules may allow swapping candidates, states have many laws about filing deadlines for candidates’ names being on the ballots or even being eligible to be counted as a write-in candidate. It won’t matter if the Democrats pick a new candidate if Biden still appears on some ballots. The courts are certainly not going to allow votes for one name to be applied to another candidate. And many electors are also bound by state law.
The bigger issue is that a switch of candidates is certainly going to peel off some votes and some participation, and this matters tremendously in a close election. If Biden had not run again or had been beaten in the primaries, a new candidate would have more momentum. Now any new candidate will face an uphill battle. And campaign finance and state voting laws will certainly play a huge factor in the outcome because even a small move in percentages will make or break the new candidate.
Regarding your confidence that a Biden replacement would appear on all state ballots, I suggest you consider that Newsweek magazine is bringing Heritage Foundation claims to the public discussion:
“According to the Heritage Foundation’s research, Georgia, Wisconsin and Nevada may prove particularly difficult for Democrats. It said that only death can remove a candidate’s name from a Wisconsin ballot, while Nevada’s deadline for changing candidates ended at 5 p.m. on the fourth Friday of June - in this case, June 28, 2024. Georgia allows candidates to withdraw up to 60 days before an election. If Biden withdrew after that date, which would be in early September, his name would remain on the ballot, but votes for him wouldn’t count. [And] some other states have no procedures to guide the replacement process, which would also open up huge areas of potential litigation.”