It is not all black and white. If I am not mistaken, it was Arsenal that knocked out Real Madrid last time in the Champions League with a total result of 5-1 in both legs. Let me remind you that Arsenal knocked out the most experienced team there has ever been in the Champions League to this date. I think that is proof enough that there is sufficient reason to believe they do have a shot here.
At the moment there is no team as dominant as PSG used to be in 24/25. PSG is not leading Ligue 1, almost lost against Marseille a few days ago and Real Madrid got their coach fired. Manchester City couldn't win the last three games in the Premier League and Inter and AC Milan are far from a perfect record in Serie A.
The only team that seems to be ahead of the rest is Bayern Munich for now, but then guess who was the only team to beat Bayern this season? Arsenal!

Barcelona looked very strong in 24/25 and yet got knocked out by Inter Milan. It's difficult to ever predict the winner in this competition, but I think considering performances in domestic leagues, Barcelona, Bayern and Arsenal can very well be considered favourites along with a few other clubs.