I think you're right, the invite window doesn't matter, but like the first 100 days of a presidency, it's a solid enough chunk of time to think about what Google+ has accomplished, and I would argue that the answer is "not much."
The invite window won't kill G+, but apathy will. About a fifth of my Facebook friends are on it, but only 3 out of a hundred post anything. A successful competitor would need to stand for something other than "well, it's not Facebook."
It may find a niche as a long-form twitter (which is something I can't believe Facebook waited so long to do) but the initial excitement has run its course.
There's a scene I really like in The Wire in which one of the major drug kingpins realizes that his product (heroin in this case) was inferior and expensive compared to the other stuff on the market, so he asks his community college professor what to do.
Rebrand it.
If they want to survive they need to hire a bunch of really, really great designers and consolidate their properties so that it looks nice, because their homepage looks like a screencap from 10 years ago. Allow people to assemble their home pages like iGoogle. Better integrate Delicious and Flickr into their other properties. Partner with Facebook to add a social component. Decide what they stand for it and make it contrast with Google. Maybe privacy. Then make that the drumbeat of everything they do.
I think Jodie Foster's character in Contact (who is based on Jill Tarter, the real-life director of SETI research) says it best.
"Look, all I'm asking, is for you to just have the tiniest bit of vision. You know, to just sit back for one minute and look at the big picture. To take a chance on something that just might end up being the most profoundly impactful moment for humanity, for the history... of history."
I'm glad they finally got rid of that extra search bar, it was ugly and clunky and useless. Any word on when this changes happens for real? I would love a better looking firebug delivery device.
This is something I've intentionally started to do with my start-up. As long as the design looks intentional, it's really not worth the stress and the bloat. Make it fast, make it good, don't make it an OCD nightmare