You seem to be saying that page load is not deterministic but that is not true. A special version of a browser can easily keep track of http requests and other async operations and consider the page to be complete when async ops = 0 and the last paint is complete. The only hard part I can think of is a page that is recursively calling setTimeout endlessly, but even that can be coded around.
Right, so like you said they have a reasonable cut-off for this one most likely ultra-rare situation. It's not going to be perfect but it doesn't have to be, in order to be useful.
