I hate this reasoning. They would do it anyway! They attempted a coup. You really think they would stop because there's some gentleman's agreement not to add more?
Trump and the GOP have always used these gentleman's agreements against the Democrats when they are in power and ignored them when it was their time. Obama did the same shit when he was in office not forcing through the supreme court appointee.
It's possible but there was at least an opportunity to try putting forth some moderate judges which would have a chance to pull over center Republicans or Sinema/Manchin. Or at least force the issue. I believe they also had a chance after Roe V Wade before election time in November when the political will was more there to defend the abortion issue by getting a supreme court more favorable on the issue