@OldToad: Actually, you're still mostly wrong. The point I'm making (which you continue to look past), is that iPhoto could be updated each time the app changed. There was little risk that an iPhoto Library would be "updated" by a later version and made UNUSABLE by an earlier version, causing major problems and lost time for the user. Why? Because the user could always update iPhoto on the older Mac and keep using the Library that had changed.
Now, however, Apple is simply not allowing Photos on anything older than El Capitan to be updated to the El Capitan version. Therefore users, including me, are allowing Photos to update their Library to the v1.2 format on shared drives, not realizing they won't be able to open it anymore on their older Macs running Yosemite or older OS's. It's a MAJOR problem for people using family sharing and shared drives across multiple Macs. It's a fundamrntally different situation than the one you described for iPhoto. You don't seem to want to acknowledge the difference OR the seriousness.
BTW, 'fragmentation' is a software industry term used to describe situations when different users are forced to use different versions of the same software For example, many Android devices cannot be updated to later versions of the OS. It's for this reason that Apple iOS fans often refer to Android as suffering from a "fragmentation problem" across the user base. It leads to many other problems and incompatibilities. It's a very good analogy to what Apple is doing by "fragmenting" the versions of Photos and not allowing pre-El Capitan users to update Photos. I'm surprised you haven't heard the term before....
Also, I still think that some updates to iPhoto did NOT render Libraries unusable by older versions of the app. Only certain ones did.