Going early is great, for sure, but not always possible, or perhaps you are holidaying and you have a day full of different locations, you don't have early start per destination, you have to do 4-5 in one day and thus some will be busy, it's inevitable.
Not quite the same thing, but I also did this shot recently and it was fairly busy, a 30sec exposure will also ghost people quite a fair bit;
Now this was just one frame at 30secs long. if I took 2-3 more I could perhaps have enough information to clone out the background people as well. It depends tho, sometimes long exposure is more problematic than a single take, because you get ghosts that trail as opposed to being static. If you take quick exposure shots, and enough of them, and the people move around, with enough frames you can probably clone everyone out, just takes more time in PP. The Composite Average thing is trying to do some of that leg work for you I think.