Hmmmm. Perhaps you'd like to point to some of these FOSS link preview add-ons. I'm not aware of any. Free as in beer, yes - there are several. Anywhere near as good as Interclue? Zip, nada, none. IMO. Also, Interclue does considerably more than previews. Whether that is a good thing or not, well, beauty is in the eye of the beholder.

Various addons (eg me.dium, now OneRiot) have had up to $20m worth of VC money put into them. Angel investment is even more common. There was a recent conference on the "Business" of add-on development. It's true that most addons are free, as in beer. Some are even free, as in speech. But there are plenty out there that have one plan or another to make money in the future. Many will fail. Some will probably succeed.