I think that's just your assumption because if you look at last season Serie A had 3 representatives who qualified for the last 8 but in the end they failed to win the title because Manchester City managed to beat Inter milan in the final match. So, it is not impossible that this will also happen to the Laliga teams that qualify for the next round because we will see how strongly they compete in the knockout round. However, they have greater opportunities compared to teams from other league. I will not measure the number of delegations from each league that qualify for the round of 16 because team from other league are quite strong this season, such as Bayern munich.
It's quite a shame that AC milan didn't qualify for the round of 16 because they are a team full of experience but they failed big in the Champions league this season. AC milan fans were both disappointed and happy because they were disappointed to be eliminated early and a little happy because they could join the European league. As said by other users, Ac milan will also easily lose in the European league because AC milan has been underestimated this season because of their instability in either Serie A or the Champions league.
Well, that could be a reversal from what they did this season. In fact, I think Inter Milan is stronger this season and they will learn from last season defeat to improve this season. The champion candidates are still held by strong teams from various leagues such as Bayern munich, Real Madrid or Manchester City and also Inter Milan. That way there is still hope because there is still a long way to go and each team must be able to perform better than what they did in the group phase.
Even for Barcelona, they cannot be considered weak because this season they have several players who are quite experienced and have previously succeeded in winning titles, such as Gundogan, who are ready to help Barcelona go further. I find it difficult to guess which team is the strongest compared to the other teams because all the teams are strong and they will compete strongly to beat their opponents.