Well, you're entitled to your opinion in terms of reasoning this way, and it means there's no need to play the game between the above-mentioned clubs, Barcelona vs. Napoli and Arsenal vs. FC Porto games.  However, sometimes we tend to think or reason in such a way and that's human for us, but that isn't always how things work when it comes to a game of football.

For instance, Since I had one terrible experience with a game, I double chanced on Metz, And that season they were at the top of the table and playing with a team close to relegation, and during the game, first half they've already beaten visitors 2-0 and 83 minutes of the game and the 94th minutes 2-2, waiting for the last 1 minutes to complete, and the visitors scored a 3rd goal that got them 3 points, that was an unforgettable loss of game for because I so much believe that there's no way Metz would lose that game, So for me, I don't use the word sure win because there's no such certainty in football and until FT,  anything can always happen.