I still don't understand why Apple discontinued Aperture and iPhoto.
#1: iCloud Photo Library
#2: Consistency with the iOS Photos app (since iOS is by far the larger platform)
Pretty much everything else follows from that. The existing iPhoto/Aperture library structure (and the apps that manipulate that library) would have needed a major overhaul to be suitable as a server-based library (multiple concurrent logins, updates pushed to all other logins, etc.). Do you make those wholesale changes to two apps, or reconsider why you're supporting two apps in the first place? And if wholesale changes are inevitable, do you start from a fresh code base, patch what you have, re-brand...? To me, it's easy to see why Apple did what they did.
iCloud meant that Apple had to consider whether they'd support the demands of commercial photographers (which is what Aperture's database-related functionality addressed) - huge amounts of storage (at a price they'd be willing to pay), incredibly high demands for speed and reliability, multiple libraries, multiple user logins, collaboration... Overall, iCloud is designed to be a single-user solution. Unless/until Apple introduces a corporate/education-friendly iCloud solution (if it ever does - they may be satisfied with IBM and other 3rd-party suppliers handling that market), I don't see commercial photographers getting what they need. Even then, they're such a small market that their needs may not be addressed. Further, this is even more difficult when it comes to video. Since iCloud Photo Library does handle video... Does Apple ever want to get into the business of addressing the needs of news gathering organizations??
Some of that might be addressed by something many who use Family Sharing have asked for - a joint photo library. It's not an unlikely occurrence, though considering the way some/many users would need to
NOT share every photo they take with the rest of the family... a topic fraught with challenges.
Serious amateurs and some pros (I'm sorta-pro - the work I shoot - travel and nature - gets professionally published on a regular basis, but I'm not exactly a photographic enterprise with numerous clients, year-round shooting schedule, etc.) may be able to get along with Photos as long as it keeps advancing - better editing tools (smart brushes will be a huge step forward - the available Extensions to date have "dumb" brushes), more ways to work with metadata, applying the same adjustments to groups of images... I don't see why any of that would be out of the question over the course of time.