I think you too underestimate PSG. Don't you see PSG performed better recently? Considering their great performance against Brest, I am not sure Liverpool will easily defeat PSG. I even think PSG can eliminate Liverpool if PSG can win in Leg 1. You must be aware that Leg 1 will be in PSG home. It will be an advantage for PSG because they will have a chance to get a good start. Meanwhile for Liverpool, it will be a challenge because they will play away. Sometimes, the atmosphere of the stadium can bring big impact on the performance of the teams.
I don't think good achievement for PSG after success defeating Brest with bigger score aggregate at playoff round match, actually the finish standings between Liverpool have unbeaten matches all league round difference with PSG have to play at playoff round match. I think Liverpool with consistent performance not only in Champion League match but also in domestic league more favorable than PSG with not quit well performance at phase league.
However all teams have been qualified to knock out round get more motivate for winning the match get higher round qualifying, if win many matches in Champion League get advantage earn much money from UEFA and its reason why many teams although not favorable for winning the match get more motivate for winning the Champion League matches.