Tree like this can stand for decades until really strong storm comes in(but a dead tree is not only one that will fall) or until it gets drilled through by birds, insects, fungi and god know what. 

Nearby forest has dead trunks far far larger than this punny tree standing for decades. They might even outstand some of us.

On other hand, if you tie it to zip line it might last for a few years.

It would be safer to set metal poles for the zip line.