It was not that Liverpool was not able to do well in other competition just that, they were just unfortunate to win the EFL Cup and beat PSG to qualify for semi finals that is football, Liverpool top the champions league table till they were knock out and they still played well to have reach the EFL Cup finals but the fact they didn't win doesn't mean they didn't do well, this is by the way already let see how next season will like for them. And for Arsenal they are sure of next season champions league again and if they can beat Real Madrid then they will go far.
It's not unfortunate that they didn't do well or were eliminated in the champions league, they lost fair and square to a superior opponent this season. It wasn't like they played against a smaller team and were luckily eliminated. PSG was their worthy opponent and they PSG proved it by playing Liverpool like one kindergarten school children in the first but were unlucky to bury that tie in the first leg at Paris. I can agree with you that they were unfortunate not to win the EFL cup.
I still don't agree that Arsenal would far I'm the UCL, let's say they managed to escape Real Madrid, their more likely to meet PSG.