Democrats controlled the Senate until January of 2015, so they could have replaced RBG at any time if she had decided to step down. As for the Scalia replacement, there's a lot of argument as to what could have been done and if they technically had a path forward, but the honest assessment of it is that they definitely didn't make a big enough deal about it and Dems kind of rested on the idea that Clinton would coast to victory.
The idea that overturning Roe v Wade will suddenly energize Democratic turnout to course correct seems like bullshit. Like, Abortion rights are already a big deal to a lot of already heavily Blue states like California, New York, Illinois, and Massachusettes... but in a lot of more Conservative places this isn't going to suddenly energize some anti-GOP Revolution. The voters in those states, especially the swing-y suburbs of Virginia, North Carolina, Georgia, Pennsylvania, etc. have made it abundantly clear that this isn't a priority for them compared to economic interests and a sense of normalcy that they crave. It's not like Texas has really seen long lasting meaningful push back within the state on their law even if there have been some demonstrations.
America is a Right leaning country marching further and further right as of recently, and the fact of the matter is the Conservative voting blocks of older individuals that can swing states just blatantly don't care and are more likely to be pulled in by propaganda and religious beliefs. And worse yet, the Senate is designed in such a way to give Conservatives more power, and the Senate controls the Supreme Court, which several Democratic Senators aren't gonna be in favor of packing even when they overturn Roe v Wade. You'd need 50 votes to pack the court, of which we might have around 40-ish now if we assume the best case scenario, and it's considered a nuclear option that is risky for incumbents so the reality is probably even worse. You need to get what, a Democratic supermajority in the Senate to have a chance of making this work. Otherwise, you're then hoping Democrats hold on to the Senate long enough to replace Breyer and then stalling until Thomas or Roberts dies since they are now the oldest (Trump's appointees are all extremely young and will be on the court for 30+ years).
It's going to be almost structurally impossible to overturn anything the Supreme Court does for the next 10 years or so, and that's exactly why Conservatives have done everything they have for the past 5 years with regards to the courts.
