I agree with you that one of these four teams for sure will be the winner of the tournament.
But I think as you said, the winner of Manchester United versus Barcelona might also win the tournament as well. Gabriel Jesus getting injured has been a big blow for Arsenal. I personally don't think Juventus have been as good as they used to be 2 or 3 years back.
So, I guess one of Barcelona or Manchester United might have an edge over the other two teams. But I think Barcelona have the most chance as United have been really inconsistent in terms of their game. On top of that they have even released Cristiano Ronaldo which is quite unfortunate. I think only time will tell who the champion will be.
Let's wait to see who becomes the champions because there are tough teams in the europa league, and let's not underrate any team in the competition. There's chance that any team can qualify or be crowned european champions. Like last season when barcelona we're eliminated by Frankfurt, nobody expected that, the results came as shock to most people because during that period, barça were one of the top levelled team with excellent performance. The ball is in all the team's court, let's see the outcome of the competition.
The presence of several big teams such as Juventus, Barcelona, Ajax and others is part of their failure in the Champions League. That way the European League competition will be even tighter after the addition of several new participants
Some of these teams still have to undergo their opening two matches before meet the group winners such as Arsenal, Betis, Real Sociedad, Feyenoord, and several other candidates.