ok, the fact that the series is going to end anyway is not a justification to let it go on for years. i don't even see how in the world that makes any sense at all. if i have to make a comparison here – bear in mind i suck at these – it's like not closing a hole in the dam justifying that all the water's going to come out anyway.
let me tell you what i think of this. if an author keeps dragging out a story, i am eventually going to think that the guy can't write an ending. and that's a horrible shame, because endings are probably the single most important aspect of any story. gosho can do cliffhangers, but i'm beginning to think he can't write an ending. and if that idea starts spreading, what ends up happening is that the series starts being called a cash-cow franchise, with all the negative implications that brings. even if gosho wants to end the series at that point – it's too profitable, no one's cutting the cord on that. ok, this applies strictly to television and stuff, but by dragging out a series needlessly, you can poison it.
let's be frank, dc has been repeating the same patterns over and over again, with the occasional dash of plot. even exceptionally written filler gets boring. it's just how it works. and when stuff gets repetitive and boring, people stop liking it. you're not just poisoning the series, you're killing your own fan base by dragging out series needlessly. i don't know if this is how this actually works – maybe i'm actually completely wrong – but regardless i don't see any good in dragging out any series any further than it should.
now, here's the thing. i don't think dc had a bad premise. i think the writing was good. i think the mystery aspect of the show was one of its key drawing points. but nothing stays fresh forever. if gosho capitalized on what he created sooner and closed the series earlier, i think it would have been much better for everybody. and perhaps, because how the story itself is shaped (3-file cases), the story is prone to getting staler sooner.
so, yes, it must end.