Why should Google agree to give away something to you for free forever? They are doing you and the majority of the users (who will never pay for Facebook/Google Plus/Github) a favor by providing the service without charging you a dime. They reserve the right to charge you in the future. If this changes, they will let you know.
I'm perfectly fine with paying for things! In fact, I'm ecstatic at the opportunity to pay for valuable online services. Did you miss the bit of my post where I exhort Google to "make this a paid service up-front"? But what's not cool is offering a means of universal personal identity for free (with the concomitant lock-in this entails), and then sneaking a clause into your TOS revealing your intentions to begin charging for this service at some future date, especially when there is no indication as to what the magnitude of that charge would be! $10 per year? I'd pay that, if my profile was especially important to me. I already pay as much for my personal domain names. $100 per year? Almost certainly not, unless G+ was the world's most important social network. Or maybe it's $X*N per year, where X is the number of unique visits your page gets per year and N is an arbitrary multiplier? We have absolutely no idea! We can't even speculate. It's a black box, a landmine, and it's executed in bad faith by tucking it away where only the most technical of users will ever notice it.
My only concern would be the link rot that might occur if your urls are later rewritten to the new username. Which makes a mockery of having the vanity urls in the first place. Having said that I can't even work out if a single Google+ post has it's own URI, or how I find it even.
