Regardless of the reason, the implementation was garbage in my opinion.
Many people don't want to lose a high information density view of their pictures.
In that article, I didn't see what I believe the real reason is that camera roll went away.
The real reason, I believe, is stupid groupthink and mindlessly following the UI trend of making everything flat and informationally sparse.
Unfortunately, Jobs isn't there to say no to Ive and Cook's stupid ideas anymore of which they seem to be spewing in droves.
Anyway, what happens when you reach capacity on your iCloud photo library? Do old pictures push the new ones off or does it just stop syncing?
Are you going to have a separate folder for things that are only in the iCloud drive? Can you tell which ones are local vs offline? Can you delete photos from just once place but not the other?
Of course, I'll probably stick on photo stream as long as I can because, to me, Yosemite looks plain, soulless, and awful just like ios7 and ios8 do.
Also, it's a downgrade if you're on a free plan as now your pictures are a part of the 5 free GB whereas the photo stream was separate.