I think that Fred Brooks said "all successful big systems start out as successful small systems". Whoever said it, I agree with it. So, I think all ULS's will grow from something smaller.
The internet is a fine example. The original WWW was not a ULS, but you can argue that it is now. The transition to web services was gradual.
Amazon started out as a reasonably small application that a small group of programmers built and understood. It was a stateless server, keeping all state in the database. There was a single database. Now it is a large collection of web services. Different ones are done in different ways. There is no central database. All this happened without ever taking down amazon.com.
It is possible to drastically change the architecture of systems. This is bound to happen to a system as it grows into a ULS. I think one of the big challenges is how a system can gradually acquire the characteristics of a ULS.