You can't simultaneously have an invite system
and a huge marketing push. It's like pretending that you are classy and telling a lot of people that they are not part of the crowd you consider good enough to make it through the front door.
You have one chance to launch a service like this for a large audience. If you want to do an invite only private beta to get the kinks out then you would probably have to do that under a different name. Once you associate the google brand with it people expect it to work and expect it to work at scale.
Anything less is blowing your first impression. 90 days to wait to try a new service? People are simply not going to wait that long, they might have tried it in the first week but three months is a relatively long period to feel left out, that's a negative first impression. It takes a lot of work to undo a negative first impression.
As for google+ not going to be killed, they did it to wave and plenty of other products that did not attract enough users to be good enough for google, why would google+ be different, especially with a purposefully slowed down launch like this? That only increases the chance.