Obama should have picked a replacement for RBG. Biden and the democratic congress should have done more to get the judiciary back in line. They had their opportunities.
RBG passed away while Trump was president. Obama did pick a replacement for Antonin Scalia though: Merrick Garland. Republican Majority Leader Mitch McConnell refused to even give Garland a hearing arguing it was "too close to the presidential election", even though Obama's term had about one year to go.