Source from Sankaku Complex (a NSFW site) :
I say that we should also consider the producers' point of view : if a title is extremely popular and its author is willing to continue it, why stop it ? Cash cows won't be allowed to get an ending just like that.
Apart from that, how long a manga should be depends on its content, so it's a bit irrelevant to seek an ideal that should be applied to all titles. For example, Death Note would have become boring and less interesting if it went over 12 volumes and applied this formula of 28~32 volumes. it's because it had a fast pace, a small number of characters and the plot couldn't last for too long. Even with those 12 volumes, some people felt it was a bit stretched. The same thing can be said for Bakuman, many fans won't like the idea of it going beyond 25 volumes.
On the other hand, wouldn't it be a waste if One Piece or Naruto ended at about 30 volumes ? Each of them has a very large setting with a wide universe and many many characters with their own stories and subplots. What I mean is that some series have an enormous potential and it is a good thing when the author is willing to explore and exploit this potential. Some few titles can drag on for too long without having their quality drop.
So the length of a title should depend on how much its content can be extanded without having the quality lower.
It also seems that manga became less defined these days. For exemple, many shounen manga who were created nowadays are drawed by women, feature a full-fledged bishounen cast and a deconstruted view of shounen elements (not to mention withe the volume length reduced compared to a usual shounen action title).