RBG refused to retire and died while Trump was president. That gave them one seat. Obama could have
McConnel refused to let Obama replace Scalia after he died. I'm not sure that had to happen the way it went down.
Sure, there may be a case here or there that would go the other way, but the vast majority of cases before this hypothetical court would be decided the same way as they have been, merely with a thinner majority.
But 6-3 is meaningfully different than 5-4. 6-3 means you can lose one from your coalition, enabling more extreme majority opinions. You can see this even in the very highest profile cases like Dobbs and Trump v US, where one of the conservatives didn't join the entire majority.
It also makes flipping the court enormously more difficult. 5-4 means that one conservative dying and an inopportune time and you flip it. 6-3 makes this statistically unlikely.
I very strongly suspect that we will see Alito and Thomas retire this year. Everybody knows how this goes now.
Yes, the Democrats fumbled this and it led to the problems we have now. I'm still a lifelong Democrat voter and always will be, but goddamnit did we shoot ourselves in the foot.
Trump had no problem convincing Kennedy to step down and be replaced. Republicans know the game, the Democrats we elect don't seem to know how to play it.
Part of me thinks this is fundamental to the human condition, but most of me thinks it isn't. This doesn't seem to have happened in the FDR era, or the Nixon era, for example. I think it's just fallout from the post-Reagan coalitions in the US political system.