Does the quality of a web interface generally degrade with the growth of market share of the business behind the web site?

I have this observation of mine, that good (usable) user interfaces on the web are usually at their best upon the launch of the web site and some time after that, and then, as soon as the business behind the web site becomes successful (if it does become such on the first place), the great UI gets gradually ruined over the course of the success of the business (especially if that is an online business amassing a sufficient number of users to make it either a monopolistic or oligopolistic player). Do you have any observations confirming or refuting mine?