The difference is that all users also eventually get the proprietary features, unlike an Open Core project under AGPL + commercial terms. I do think Fair Source is a better model than Open Core, at least in most cases, because of this alone. So I guess, would you rather: 1) never have the proprietary features, or 2) have 2-year old proprietary features? I know what I'd prefer, and from a simple continuity perspective, I know which is preferred by users.
Like I said, I'm not saying AGPL is bad. I just don't like how it's used in startup-land and I think there are better, more honest, options now.