I was getting 10 hours of usage and 15-20 hours of standby time on iOS 10.That is not bad battery life. Your expectations are totally messed up.
15 hours standby time is more than enough time OFF the charger to get through a day. For example say you take it off the charger at 6am. 15 hours since the last charge puts that at 9pm, with 19% battery left. Which could easily last until midnight with some use. That’s pretty much a full day. You’re confusing time since last on a charger vs actual on screen usage time. You’ll never get 15+ hours of on screen usage time.
Your expectations are off, not the battery life.