I think G+ is kind of Google's Internet Explorer moment. The have dominated search for over a decade, but the realised that social media had become massively important and that they just weren't relevant there. They couldn't acquire anyone large enough and their previous attempts at entering that area basically all failed. So in the end they had a choice of either giving up and risk being overrun over the long term or they could do what MS did with IE: Leverage your dominant position in one field and force your new product down your existing customer's collective throats.

The difference is that it worked for MS (for a time at least), but so far it didn't really work for Google and instead their existing customer base is getting more and more annoyed. However, at this point G+ is so deeply integrated that a graceful exit from this strategy would be next to impossible. Google is basically all in with this.