Comparing virtual goods sales to premium web app conversions is a bit of a stretch, but I happen to like Nabeel, and I think that link is a good article.
I can absolutely see your point, but I tend to see them as two branches of one tree- You're inviting people into your service for free, and then trying to sell them a higher quality version.
The difference is that one transaction is a one-off sale, and the other is a subscription.
It's more then that though. People subscribe to web applications for completely different reasons than people whom purchase virtual goods. The demographics, spending patterns, motivations to spend, lifetime value, upsell opportunities and a million other things of the customers differ. Comparing money spent on entertainment and money spent on tools isn't a good idea.
Overall, Freemium conversion rates are way down right now. I might suspect that users are feeling less wealthy, and not opting to pay for anything that's not absolutely necessary.