How could I know that basics OS features like drag and drop won't work after updating photos? Is that something a user expects? I don't think so. Because that never happened with any updates of OS and iphoto before.
It happened before. Every once in a while macOS gets completely redesigned and legacy code is dropped, to make the system small and fast again, adapt it to the new hardware and processors, include new security features. These major redesigned are disruptive for the compatibility with older applications, because the foundations they are built on are no longer available. And even if they still run in some way, they can no longer interact with the the new system.
It happened with the transition from Snow Leopard to Lion, when the support for PowerPC applications was dropped, colorful icons vanished, and Spotlight changed considerably. We now have a Spotlight that can direct us to the next movie theater, but is nearly useless to show us the path to a file on our Mac.
And Yosemite brought the next major redesign, with a strong focus on Cloud integration, Full Screen apps, new security features changing the interaction between applications.
If you were using all apps in Full Screen mode, like Apple expects the users to do, the missing ability to drag and drop would not matter, since there is nothing to drag to. But I never use any app this way. I want to see more than one app at once and my Desktop as well. Otherwise I could use the iPad instead.