You're right, it's too early to conclude that Barcelona won't win more than one trophy this season, and it's also too early to conclude they will. Competition is still very tight in La Liga and also in the European League, it will not be easy for them to win the championship which is still very long. Also other teams will not allow any team to win the title easily. Challenges are still waiting for them ahead.
Football is unpredictable its not yet sure for any club to win lose. This stage of the game anything can happen. Will feel team like arsenal can win the trophy, it can be a surprise if the Europa game starts arsenal performance may change. Bad performance can occur to any team at anytime and a team can improve anytime as a surprise to anyone.
I am pessimistic that Arsenal will be able to win the European League trophy this season because bad news is hitting Arsenal, one of their mainstay players (Gabriel Jesus) is injured and reportedly it is a long injury
As you said, football is unpredictable, even a small team can bring big surprises. there are barca, juve and mu who are currently competing for the european league trophy and i think it is very tough competition