You can shout "Mitch McConnell" and "Senate Republicans" until you're blue in the face; democratic leadership lecturing about the importance of the presidency in order to control the Supreme Court and then letting Scalia dying while Obama is president turn into the appointment of Neil Gorsuch was a collosal blunder that damaged not only our welfare as a country but permanently scarred the credibility of the Democratic Party.
The Constitution says that the President "Shall appoint" Justices with the advice and consent of the Senate. What McConnell did was Constitutionally questionable at best. Democratic leadership could and should have made it an issue but failed to do so because they believed they were getting the better of Republicans.
They believed this because of a severe overconfidence in Hillary Clinton and the platform she represented, and that overconfidence stems from being completely out of touch with severity of problems most people are facing.