Coming from a 6 Plus here, upgrading to a 7 Plus
All the normal reasons people upgrade:
Better processor
Better battery life
Dual cameras look awesome
I like Apple stuff
I don't think the average person is upgrading cause of better processor, and the camera bump looks pretty awful on the 7 Plus, to be frank. More area to get scratched or dirty. Huge size means you're going to need a relatively thick case to keep it from getting scratched on surfaces, etc. which means the phone is going to be thick and heavy AF when you run with a case on it - which has been my issue with the 6 and 6S Plus over the past couple of years.
The phone is nice and thin/light with nothing on it,but it's a bar of soap. So, in order to add grip and keep the camera bump off of surfaces you end up bloating the weight and thickness of the phone considerably (as well as it's already overbearing WIDTH - thanks to HUGE bezels).
I also don't think Apple's "improved battery life claims" have ever really materialized. They've been saying you're going to get 1 more hour of battery life for years now. If they were actually delivering that, then small iPhones would be getting 8 hours of SoT at this point. They clearly aren't.
They've stated higher battery life (about 1 hour or more) for at least 3 different iPhone releases and at least 1 (maybe two) iOS releases in the past few years... Battery life continues to be rather terrible.
It's time of them to start using bigger batteries. To be honest, I'm getting exasperated with these big phones, which are the only solution due to battery life on the smaller ones being as bad as it is.
With the iP7 and iP7+ having the same main 12MP camera module, there isn't really a compelling feature for me to consider the Plus, except the terrible battery life the 7 assured to have in comparison to the Plus - and many other phones in the 4.7-5.1" size class (as in, practically every other major flagship/release in that category, even mid-ranged devices).