In my opinion, no result at the group stage can be considered unexpected - for top teams, this is the second (if not third) priority. If a serious team loses points in the group stage, this will not change anything - it will still get to the playoffs (even if through an additional playoff). But if a serious team loses points in the national championship, this can be critical for the entire season. Therefore, at the group stage, weak teams (for whom participation in the Champions League is not routine) have increased motivation and can beat stronger teams that lack motivation.
I will not try to dispute this point of yours as it's very much valid but then I will like to call your attention to another aspect of these teams, which I believe id you carefully consider, you will see reasons as to why to an extent the results this big teams get at this stage of the champions League does matter, one of them is the psychological effect it will get in them and how its going to affect their confidence throughout the champions League tournament, and you may ask me how, if a team has recorded poor results in this group stage, trust me their minds can go messy based on the priority places on the champions League, they can aswell loose confidence facing bigger teams beyond this stage that's if they get passed it, so for me every stage matters a lot a s has got a lot to do with their chances of succeeding in the league mostly beyond the current stage.