They were contracted to make five albums...the fifth one drops in November. I didn't see them sticking around much longer after that. I did think that they would tour the following summer, and was originally surprised to hear that part but the more I thought about it...
Their US Tour is not performing very well. I've already been to two shows, and at both of them the upper bowl of the stadiums were practically empty. I'm seeing them again next week...and we bought the tickets through Groupon. I think that speaks for itself.
This has happened to numerous bands (boy- or otherwise). *NYSYNC, Backstreet Boys, Spice Girls, the JONAS BROTHERS, etc. Not even blood could keep JB together. The only band I can think of that has stuck it out is Hanson...and again, blood has a lot to do with it. But if you watch or read interviews with Isaac, Taylor, and Zac they're very open about how they've almost called it quits multiple times. I truly didn't think 1D would last.
They're already involved in side projects and other things they love. Harry's been writing songs under pseudonyms for a while. Niall's always out golfing with the pros. Louis is hosting beautiful charity events, and Liam's doin' his thing. It's totally natural for them to want to continue to pursue their own ventures.
Zayn already left! We know from previous experience (aka the Spice Girls) that once one member leaves it's just a matter of time before everyone else splits. There's still hope for reunion tours (I'm looking at you NKOTB and BSB), but the end is coming.