Well, there are some significant negatives to a powerbank solution.

• Charging from a powerbank takes a long time. Swapping a battery for fully charged one can be done in half a minute.
• You need to make sure the cables stay connected. How many times did you have phone on charge in a backpack for an hour, only to find out it wasn't charging because the connection got a little loose?
• Each charge cycle reduces lifetime of the battery. If you have two batteries to swap between, you effectively double the overall lifetime.

And of course, when (not if) the battery dies, you can replace it at your leisure, instead of searching for a repair shop in the case of phone being glued shut. Battery is pretty much the only perishable component in your phone. If you keep your phone safe from falling or drowning, it can work just fine for decades. But the battery will need to be replaced one day, regardless of how careful you are. You also wouldn't need to pay the extra service fee if you can open the phone and put the new battery in yourself.