Apple calls the new watch "Apple Watch", and now they replace iPhoto with "Photo". Why doesn't it stick to the iNames? This is confusing...
Apple has been phasing out the ‘iProduct’ naming scheme for a couple of years now. iCal, iChat, iPod (the app) have been renamed to Calendar, Messages and Music. Photos just follows this new convention. I suppose it makes sense for Apple to have simple and clear names for applications that are shipped with OS X.
A lot of people here seem to be missing the fact that Aperture really wasn't making the money it needed to stay viable anymore.
Otherwise Apple would have kept it and only killed off iPhoto. Additionally, to substantiate my point, if Apple were solely thinking about the profit, if Aperture was making any money for them, they would have kept it and integrated iCloud Photo Library into it, then they would have made a bucket load with the cost of Aperture as well as the money from iCloud Drive storage. The fact that they're killing Aperture off just shows it wasn't worth keeping.
I have another suspicion: reallocation of resources. Apple saw the potential for a more powerful Photos application after they saw the interest in Aperture even among consumers, enough so for them to throw out iPhotos completely and incorporate advanced features. I think they just don’t have the developers to maintain two applications of that kind, especially when their features overlap and both consumers and ‘prosumers’ are happy with Photos. The professional-user segment would be further marginalised.