Real Sociedad was lucky to only meet Inter as the only strong team in the group stage, but if Sociedad were in group F then I am sure Sociedad would not be able to win the group stage or could even be eliminated from the Champions League.
Not all teams that are undefeated in the group stage and become group winners will succeed in qualifying for the quarter-finals if they have strong opponents in the round of 16. PSG is a strong team and will not be easy to beat, so it would be very unfair if you ignore PSG's chances without any consideration. Real Sociedad certainly has a chance to beat PSG, but PSG's defense is not as weak as you mean.
Despite lucky or not, Real Sociedad success managed top standings position in group phase and Inter Milan have to finished on second standings position, I don't sure with Real Sociedad qualify to the next round depend on lucky because in their group only Inter Milan is strong opponent teams. Actually most lucky teams qualify to 16 round is PSG because has the same points with AC Milan but help with goals productivities.
PSG is not strong team in Champion League, they losses from Newcastle United, AC Milan and losses compete with Dortmund to finish on first standings position. Real Sociedad will be difficult opponent team for PSG on 16 round although many people favorite with PSG has large opportunity qualify to quarter final round.