Web properties, just like any property even in the physical world, become more expensive to operate in the long term. Very rarely do I hear of companies spending less money to operate something in the future than they did early on. Part of this is scale, as things become more popular, it takes more people to keep it running. Another part of it is macroeconomics, stuff they rely on, particularly labor (landscapers in the physical world or server admins, for instance) can become more expensive due to a number of factors. A lot of people, like the author of this post, assume that once a web property is built, it exists forever (like a statue or monument), but that's of course not the case.
When a company needs to make decisions to increase their margin, a lot of people attribute it to "growth," and use it almost in a derogatory way. I think a more charitable way to see it is that it's not necessarily growth (although I'm sure that is a factor), but just to even keep the website running, a lot of times the same decisions need to be made (i.e., layoffs, charging for stuff that used to be free, etc).
When you think about it this way, it really seems like either these sites make decisions like this to continue to exist, or they cease existing. This seems to track pretty well with a lot of cases I can think of. I think some very ideologically motivated people can work extremely hard to keep operating costs down to avoid this scenario (Craigslist comes to mind), but this is not easy, and it's very far from being the "default".