Apparently, you haven't heard the conservative argument against that, because if you had you probably wouldn't be saying what you are. In 2020 Republicans's had the white house and the senate which means that they can push through a new judge. Four years before that, Democrats didn't have the senate and so it would have been pointless to try and put a new judge through.
It's a simple as that. A president is elected for FOUR years, not three, and if he has the senate, he can put a new judge through. That is the constitutional process regardless of RBG's SUPPOSED dying wish.