You have to consider the context. If it had ended on its own terms and passed the torch to other Arrowverse shows, that would be a satisfying ending. But the Arrowverse has been abruptly gutted due to corporate shenanigans, and The Flash was the only series left in that reality. So a whole thriving shared universe has unexpectedly collapsed in a very short time due to external factors, and that's frustrating.
I feel just the opposite. Season-long arcs aren't enough for a superhero show. Superhero series should be about the heroes helping lots of different people and stopping a lot of different threats, not just butting their heads against a single problem for 12 weeks until they finally crack it in the finale. Solving procedural cases of the week isn't "filler," it's what should be the meat and potatoes of a superhero series, because then the heroes actually get to be heroic, actually get to help people and score victories. I vastly prefer The Flash's approach of strong episodic plots with a larger arc building in the background to Superman & Lois's pure serialization, where Superman comes off as ineffectual because he's failing over and over against a single bad guy until he finally ekes out a win.
