Earlier quoted context omitted.
Google seems to have a lot of internal silos. The same can be observed in (historic?) Microsoft, current IBM, and other corporations. Perhaps the split into Alphabet with Google, et al. as subsidiaries will help? But I doubt it. Anyways, my point is: Someone has a good idea, they implement it, it reaches the point where it's not easily integrated into another product. Instead of strong corporate leadership directing…
It seems like someone said, "why don't we have a WhatsApp competitor?" and so Bob and Steve made it without thinking about all the different messaging applications already under the Google brand.
G+ introduced "Circles". I really liked that concept, it was a smooth way to organize the people I wanted to communicate with. Post a message, only my gaming buddies could see it. Post another, friends and family (and most gaming buddies fell under friends) could see it. And it was under my control and invisible to other users.
Talk: Adds nothing, it's just another messenger.
Hangouts: ... video and audio chat. Screen sharing.
Spaces: Seems like a more socially oriented Wave?
Wave: Neat, got stuck in performance hell and was hard to sell. The invite system didn't help. Collaborative documents. Integrated chat bots with the document much like this @google thing in Allo seems to be.
Allo: Talk - gmail account + (end-to-end encryption OR @google chatbot). It's WhatsApp meets part of Wave meets Google Talk.
Take G+, integrate Spaces into that. Make Hangouts part of that. Make Allo a part of Hangouts. Move Hangouts to end-to-end encryption model (broken with @google chatbot present, but not if you leave it out).
[0] I'm excluding GMail because it, at least, has a fairly well established place on its own and is, fundamentally, an email platform. The accounts are tied to the other services, but GMail is easily its own standalone product with a very large user base.