I think this is a discussion based on something that couldn't really be avoided. The legal (typical US litigious mindset) situation, as I understand it, was that the funding for Biden's (Biden/Harris) campaign could NOT be legitimately transferred to another candidate. When Biden agreed to stand down it was too late to start that funding from scratch, so the only reasonable option was to appoint Harris - since Harris was already explicitly on the funding ticket, right there in black and white (Biden/Harris) the gathered funds would be legitimate to continue to draw. This was obviously far from an ideal situation, but the Democrats had to weigh up the pros and cons of:
Sticking with Biden (legally safe, but knowing that this was highly likely to deliver defeat);
Transferring to Harris (risky but better bet than Biden, and best to avoid legal challenge);
Appointing a new nominee and transfer funding (risky - time-consuming, with big chance of falling foul of legal challenge);
Appointing a new nominee and starting with a zero balance sheet and fund from scratch (best chance of securing the most ideal candidate, and request donors to transfer their donations - but the legalities of that were unclear at the time and probably highly challengeable).
Of the above 4 options, each having significant downsides and legal implications, the Harris option was selected. And it might've worked except for those pesky voters who care far less about the fact that Trump is an adjudicated rapist, a multiple felon, known liar(*) and instigator of a violent attempted coup... and care more about the fact that the lies (which they know to be lies) actually appeal to them.
(*) Trump complained that FOX would not repeat his lies on air this time around - but that was because FOX were fined almost $800million dollars for transmitting Trumps lies last time, so were clearly not so inclined to fall foul of this repeating of known lies this time around.