Getting photos out of old iPhoto libraries has been a nightmare for me. I will never use a system that is not a simple directory structure for photos again.
In addition to that, I find that photos are actually harder to manage when everything is a singular blob on all of your devices. When trying to free up space, I am not sure what can stay and go and what is safely stored elsewhere. I actually moved my parents from Photos to Dropbox for this reason (and that they are on Windows). It is now trivially easy to just erase all photos on anyone's phone and be confident it is living in an easy to find directory structure. I no longer have to spend hours making sure that the photos I think I have backed up are indeed backed up and nothing will be lost when Photos are taken off of an iOS device.
I'd rather have access to a file system that has them and I can sort through that way, do copies, backups, etc. without having to touch a 20+GB file.