I think after having thought about it, it’s quite a good ending imho
I quite like the whole idea that their obsession over there being one lynchpin that they could catch and bring to justice was just a bit naive. It’s a more realistic ending that they’d spend so many years fighting against the tide only to pretty much fail and be shut down, there’s no way that an institution with proper institutional corruption would allow itself to be caught by its own anti-corruption unit. And to end up effectively neutralising whatever small amount of power the anti-corruption teams had seems an appropriately realistic ending.
Of course, in TV terms it results in an immediately unsatisfying ending, and definitely it was probably affected by COVID, but I think it’s quite effective in conveying the message that we live in an inherently corrupt society where the “good guys” don’t win.
Also whether or not that’s the “point” of Line of Duty is a totally different question.