A lot of people never plan an exact ending for their work because, as a few have mentioned so far, a series is often planned to continue indefinitely as long as it can maintain a readership or bring in money. Even when you see a creator claim they had an ending planned from the very start, looking at the work can tell you whether they really did or not (hint: most of the time it's 'not' XD)
Speaking personally, I think it's important to be flexible. In the course of writing a story, sometimes characters change from what they're expected to be, or events seem like they'd be better if they went a way very different from how you first imagined. Sometimes big things need to change, and if you have too rigid an ending planned from the very start, it leads to additional work that shouldn't strictly be necessary. If your story changes and feels like a living creation, you really need an ending to reflect that.
I tend to prefer open endings for this reason. It usually comes off as implausible or disingenuous if everything is tied up too neatly or, worst of all, everything "goes back to normal" and no one ever seems to have a problem with this or ever has to deal with what they went through.
It's so true! A lot of mangaka, of course, have assistants to handle the tedious stuff that the creator doesn't need or want to be bothered with, but they still have to do great masses of work. Though unless it's a weekly magazine they work for, fortunately their workload is much more manageable. Weekly magazines are monstrous though. High potential return, but that doesn't do you much good if you've exhausted yourself into burnout or poor physical health.
