McConnell has already packed the federal judiciary, beginning in 2015 when he shut down all federal judicial nominations made during the last two years of President Obama’s administration, building up over 100 judicial vacancies for Trump to fill upon taking office in 2017.
In fact, one of those nominations was Myra Shelby, nominated in January 2016 for the vacant 7th Circuit Court of Appeals. Amy Coney Barrett filled that stolen seat.
So, if the Democrats were to pack the Court(s), it wouid be perfectly consistent with what has already happened since the GOP took control of the Senate following the 2014 election.
Hence, your premise is flawed.
Link: https://www.brookings.edu/blog/fixgov/2018/06/04/senate-obstructionism-handed-judicial-vacancies-to-trump/