Regardless. The Dems enjoyed 8 full years of Senate majority, at points a very healthy majority. The window was 2007-2014 and they did replace two Republicans (Souter and Stevens) with two very liberal Democrats (Sotomayor and Kagan). The GOP's window only started in 2015 and will likely end in 2022 when Ron Johnson, Portman, Toomey etc. are up and have to defend losing battles.
Each side gets their eight-year window.
SCOTUS deaths are random but let's not forget, Obama did replace TWO Republicans, while Trump has also replaced 2 Republicans but no Democrats yet up until this point.