Anything can happen, we can see how Real Madrid with 3x UCL trophies in a row then the possibility of Manchester City will fight for the UCL again but certainly we cannot determine that exactly because in the UCL competition it is still difficult to guess even Real Madrid or other teams.
Of course, no one knows which team will be the champion of UCL this season. Although Real Madrid and Man City are the strongest candidates, other teams also have the chance to win UCL as well. Sometimes, UCL result can be very unpredictable.
Is it true about that theory? Whoever eliminates Porto they can win the UCL? I've only heard this theory because I honestly didn't notice when the winning team beat Porto first.
It is not a theory, it is purely a coincidence. There is nothing to do between winning opportunity and beating Porto. Porto is not the strongest team, Porto match never guarantees the trophy. I don't understand why many people assume the winner in the match against Porto, to have the biggest chance to win UCL trophy.
Who knows, Arsenal is not the favorite can also happen this season with the title with Arteta.
Arsenal also has the chance to win UCL trophy. But I'm not sure if Arsenal could beat Bayern Munich if they play with their best. Bayern Munich has a great squad and they an experienced team in UCL, too. TBH, Bayern Munich is one of my favorite teams to be in semi-final round.