Manga has it worse in the sense that unlike the monthly or bi weekly western comics, manga is more times than not a weekly thing. They lack the massive teams of the western books and have to constantly keep things rolling to keep the masses short attention spans in a heavily saturated market. It's an up hill battle for sure- however it feels that the “ spirit” of a series, it's main driving focus seems to fade rather swiftly. Off the top of my head I can name at least 5 or 6 running series currently out that seem to have lost their way- or well on the path to becoming a utterly different series with the same characters.
How do we approach these as a reader? That is up in the air. Fans will come and go, some loose interest and walk off yet there is always the possibility to gather new readers who may have not been fans of the first set of arcs but in love with the newer ones. I think it's sad in a way when a series out grows it's own roots, and seems to just be “ reaching” to turn in a weekly manga. Worse yet if the Mangaka already admits to a short attention span themselves. This usually leads to a “ fill it out as we go” mentality that will very quickly loose focus.