Slight correction: FDR was not successful in packing the court, but he was making plans to do so since they were striking down parts of the New Deal as being overreaching. This was met with overwhelmingly negative response from not just Republicans but also Democrats as well, and FDR was painted as an attempted tyrant who was going to irrevocably alter the structure of the court just to get his way. This led to the Republicans gaining tremendously in Congress after being reduced to less than a quarter representation at the height of FDR's presidency. They almost doubled the number of seats they held in the House in the 1938 elections, from 88 to 169, and by the 1942 election they gained another 40 seats, reducing the Dems to a slim majority. While the Dems would continue to hold at least a slim majority in both chambers for much of the following 50 years, FDR's court packing plan put an end to their overwhelming dominance in the early part of FDR's presidency.
Truthfully, court packing should be electoral suicide for anyone that entertains the notion. It's easy to see why nobody should attempt it: simply put, there's nothing to stop the opposing party from adding even more justices when they come into power, then the original party from doing the same, and so on. But the Dems' base has become so rabid and short-sighted that they're demanding it, and Joe is too senile or dumb to quash the notion outright. Hell, not only did he not stamp out any rumors of court packing, as you pointed out, he's flat-out stated that the public doesn't deserve to know his stance! How the fuck does anyone think this is a good thing to say?
I'm gonna be immeasurably disappointed if Joe fucking Biden successfully cheats wins this election. If there are seriously that many people who are okay with just tearing up the Constitution and doing whatever the fuck feels good in the moment, then our country deserves what it gets.