First: Why do you think they created a completely new app rather than integrating features into the gmail app one by one? I understand it has a good integration of Google Now in it, but Google Now itself works very nice for me (Like showing important stuff from my gmail). Thanks!
Every time they add features to gmail, people on forums like this one scream "OH MY GOD GOOGLE LITERALLY KILLED MY SISTER". It's a lot less irritating to make major changes in a new product.
I'm not a PM so I can't speak to the decision itself, but I don't think grafting on the Inbox workflow to the Gmail interface would have worked out. It would have made both of them very muddled, and given the impression Inbox has more flexibility that it really wants to give.
Two apps using the same backend data (you can switch between Gmail and Inbox if you like) seems a better fit.
> Why do you think they created a completely new app rather than integrating features into the gmail app one by one?
As I see it, it lets people wanting to try the new thing do so, but has zero effect on people happy with what Gmail does now who don't want to be distracted either by more option clutter or the features themselves.
Further, it lets people wanting to use the new workflow do so without clutter from the regular Gmail interface (and being a more focussed interface is the key "interesting feature".)
It's too different from Gmail to be integrated into it. It coexists with Gmail so that you can choose to use Inbox and its "opinionated" workflow, or not. Users who like their personal Gmail workflow probably wouldn't appreciate having that flexibility taken away if it's not aligned with how Inbox wants you to work.
Because for many people that would be the final straw to push them out from Gmail, leading to them leaving Google completely, since once Gmail is gone, there's hardly a reason to keep Google account anymore.
From ethnographic research, this is a good move by Google. People enjoy a blank slate or 'fresh start'. By starting from Inbox 0, people can enjoy that feeling. Contrast this with integrating into Gmail where people have thousands of emails in their inbox.
