I'm sorry but that is simply not true. Apple announced the coming of Photos for the Mac months before the app was released, they explained why, integration with Photos on iOS, the wonders of keeping all your photos in iCloud and all the rest of it a long time before it happened. Various pre-release versions were made available for the adventurous too as I recall (I wasn't adventurous). From memory it was about six months (at least four anyway) before Photos for the Mac actually appeared following the announcement (many complained about how long it was taking). There was endless discussion of it on many of the websites I listed previously, and many others too.
This is no doubt true for people who follow closely what Apple is doing, and are engaged with the ins and outs of how software works on a day-to-day basis. But it wasn't true for me.
Photography is not my main activity (that's writing, whence my presence on the Nisus boards), and I really wasn't prepared to have to take time off to learn a new way of handling my photos.
I knew that IPhoto was due to be replaced by Photos, but I assumed that either the basic commands I was used to would remain the same, or that I'd be offered an easy path to switch from one program to the other. Neither of those things happened.
For me the change came when I bought a second Mac that came with Yosemite, while my main machine was still on Mavericks. I used Time Machine to migrate my data from the latter to the former, and was surprised to see that during the initial period, IPhoto still displayed on the new machine.
Suddenly, without any prior warning as I recall, all my pictures had been migrated into Photos on my new machine, and I instantly found myself unable to function as I'd been used to doing. I suspect—but I don't have forum messages to prove it—that a lot of other casual users of IPhoto have had similar reactions to me.
I think there are two root causes of this problem: one is that of all software packages that are bundled "free" with computers. The provider obviously has to ensure that they at least work, but they have no real motivation to cater to the needs of each and every user. That's one of the reasons why I appreciate Nisus; apart from the fact that it's a superb program, the money that I've paid for it means I can make demands, ask questions and receive replies.
The second problem, I suspect, is specific to a company like Apple, which is simply far too rich for its own good, and probably making too much money off IPhones to care all that much about computers. But that of course is only speculation; it may not be true.
All that having been said, I intend to make a try at adapting to "Photos": no doubt it's pretty simple once one gets into it. But I do resent having to learn, and then unlearn, things every few years!