There was never any room for Plus. instead of recognizing a subset of users who enjoy social media and offering a better product, Plus focused on offering the same product. Then, when it didnt become an instant sensation, they threw a tantrum and made all users social media users by embedding Plus into everything that google did.
I actually think a big part of the failure of Google+ was something that, in hindsight, looks so small that a lot of people forget about it: When Google+ launched, it was a limited invite-only service.
Google had previously had good experiences with that sort of limited/phased rollout, particularly with Gmail. The fact that it was hard to get an invite helped generate hype for Gmail, and I suspect they were hoping that creating the same kind of artificial scarcity would help Google+ accounts to become equally sought-after. And it worked, for a little while. There was a brief period of time where lots of people wanted account, and they were nearly impossible to come by.
However, whereas Gmail users can continue to communicate with people who use other Email providers, the utility of having a Google+ account is directly related to having all of your friend on the same social network. Because of this, in hyping the service by limiting the availability of accounts, Google was shooting themselves in the foot. At the time of greatest hype, right when the early adopters and people who are social networking hubs would be most eager to try the service, they either weren't able to get an account, or else they got an account only to find that their friends couldn't get an account. In the very important window of time between when Google+ was launched and when people had made up their minds about it, it had already earned a reputation as being "possibly potentially good, but useless because no one is on it."
And that narrative just stuck. A social network with nobody on it is of no use to anyone, so the narrative became a self-fulfilling prophesy. Nobody ever bothered using Google+ because everyone already knew that nobody used it. As Google started to realize it was a failure, they then tried to force people to use it by linking it with all of their other services, but they should have known better. The harder they tried to push people to use it, the more of a backlash it created.
Remembering back to the time, there were a lot of people who had become frustrated with Facebook, and I think that it would have been possible to get a substantial user base simply by offering a viable alternative. Unfortunately, Google tried the wrong marketing strategy, generating hype by limiting availability, and it backfired spectacularly.