I went to Barnes & Noble the other day with my friend Dave. We were walking around aimlessly, checking out random books, when we came across the calendar section. He picked two and asked me which one he should get.
"But you already use Google Calendar," I said. "Why do you need a paper version?"
"Oh, I don't need it," he replied. "I just want it."
Here is a guy who is an early adopter of every technology out there, who tries the beta version of every new service he finds and then integrates it into his life. And he still wanted a paper calendar.
Lesson: Some products are timeless. They stick around long after technology has seemingly made them obsolete.
First, paper calendars have more than one use. They don't just show the date. They also take space on your wall with pictures that change every month. The first can be replaced by Google Calendar, but the second can't (at least not yet!).
Second, the paper calendar is there to complete or complement an existing timeless and static thing*: a wall. As long as there are walls, there will be posters and paper calendars to decorate them with.
Similarly, as long as cars have bumpers, there will be bumper stickers. But just like paper calendars, they don't just take space on the bumper. They are also a channel of self-expression.
As long as there are tables, there will be table clothes. But they don't just decorate a table. They also protect it from foodstuff and the elements.
Formula: As long as there exists static thing X, there will be <your product>. But <your product> doesn't only <verb> X. It also has <alternate use>.
If your product fits this formula, you can be sure that it will be around for a very long time.
Note: by "static", I mean "doesn't become more advanced or change in functionality". Computers for example don't fit this formula, so your piece of software will probably not be timeless no matter how you design it.