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I don't know why you just concluded that Arsenal can't win the Champions League next season. I can also say Arsenal can win the Champions League next season. But if we were to mention the five teams that are favorites to win next season’s Champions League, I believe that Arsenal will be included in the list with their current performance. Arsenal is not a weak team that we can just conclude now that they can't achieve such a trophy. With the way Arsenal is beating their opponents currently, it is very possible they could fight for the Champions League next season.
I will not ignore Arsenal's potential in the Champions League next season, it's just that you have to admit that Arsenal's probability is no better than Real Madrid, Manchester City, Bayern Munich and several other top teams. Arsenal don't have a strong mentality when competing in the European arena like the Champions League, so I don't expect them to jump too high next season.
It's up to you how you convince yourself about Arsenal in the Champions League, I don't think Arsenal is mature enough to win it. On the other hand, I think Manchester City will be enthusiastic again next season in the race for the Champions League title besides Real Madrid, so I will put Arsenal in 5th or 6th position on the favorites list.
Considering how impressive Arsenal have been in the last two seasons especially in the English Premier League, I think it'll be absolutely disrespectful for anyone to ignore or underestimate them and how far they can go in the UEFA Champions League competition come next season but like you rightly said, Arsenal's probability of winning the European Cup next season isn't as high as other clubs like Real Madrid, Manchester City, Bayern Munich or even Inter Milan. The Gunners despite their impressive form in the English Premier League in recent seasons haven't been able to reach the semi finals of the Champions League for over 10 a decade now.
However, despite not reaching the last four in over the last 10 years, I strongly believe that the current Arsenal team if they continue with their recent impressive performances can make it to the final. All they need at the moment is to believe in themselves that they can do it and I see them going far in the competition.