at some undisclosed point of time in the future, after having left professional users hanging with only some bare-bones app for a significant period. So just like FCX, Pages, Keynote, Numbers
1st off FCP X is a much better application then previous versions of FCP. Yes it started off rocky but Apple stepped up and over the initial year added missing features.
Same goes with Pages, Numbers, & Keynote. Apple has been constantly enhancing those apps. Yes, they start off with the most basic and needed features and they build out from there. One of the most requested features I have seen on the forum for aperture was for a iPad App version. In addition, other things that are involved are backups/storage options. Not to mention the process to use a library or even a project on multiple machines. Like Pages, Numbers, etc you need to start from the ground up with a foundation that works across all devices. One that operates the same across all platforms.
3rd party access is there from day 1. Apple already showed this off at WWDC for iOS. Thus, the ability for 3rd party apps to be used within other apps. i.e using filters, editing tools, etc all from within the photo's app. Or, you could use a 3rd party app and access you photos library. All with non-destructive edits (ala Aperture). All now saved to iCloud, sync'd across all devices and edits that appear immediately across all devices. Same would be true on the OS X end.
I also think Apple has learned a little from the intro of FCP X as well as the new suite of Productivity Apps. They are still developing photo's, per WWDC. Thus, by telling us now that Aperture is no more, they can field responses, such as these forums and submissions as to what other hope is not missing and maybe have time to make the most requested part of the initial release. Maybe they will also be more vocal, after the initial release, as to if more features will be rolling out over the year. Which is what happened after the initial outcry from the productivity apps release.
Oh and don't forget that no one is taking your Aperture copy away. So, you loose nothing. It will be updated one more time to work with Yosemite, so you can still do all the things you wanted to, just like using the previous version of the productivity apps. No one is forcing the change.