Liverpool are still empty-handed...
FA Cup : eliminated very early
EFL Cup : eliminated in the finale
Champions League: eliminated during last 16
Now there is only the Premier League left to win. We can already call them as the champions here though. Because the gap is 12 points with Arsenal so I don't think even Arteta believes in their chance anymore.

Do you call Arne Slot successful for this year? I think winning a Premier League trophy is a good start at least. He is about to end Liverpool's thirst for this title -
the last one achieved was in the 2019-20 season.
I do and why not? I know it's painful to see how his team still empty handed even when his team still at peak performance, but their chance to win PL is just getting even closer. If his team can win at least a trophy this season, it can be considered as a very big achievement.
EPL is far bigger than EFL, and not so many managers were able in winning it. Look at how Arteta has been managing Arsenal for more than 5 years, but never win PL.
Arne is doing big as long as he can win his first PL. It's even bigger than Arteta's whole career at Arsenal.