True, but I was being a little ham fisted in my original reply. They did win out by executing better than their competitors in the competition of the moment, but what I was trying to suggest was that, in all probability, no-one could have succeeded in trying to do those things at an earlier time. You couldn't go back to 1996 and start YouTube.
With YouTube you couldn't have survived the storage costs at an earlier time, and also there needed to be ubiquitous playback, in the form of Flash. Flickr similarly would have been financially onerous due to storage costs ealier on.
In 1996 our bandwidth was so tiny that you really could not have done YouTube at that point.
Wikipedia is a slightly different point. It was launched in 2001, so in 1996 you obviously could have gotten a lead on it, but if you look at the growth curve it wasn't until 2005/2006 that it really took off. I just don't think that the community of editors/users was broad enough until that point.
You are totally right that good execution counts for most of the success and going back in time would allow you to do that much better with the hindsight gained. My thought is just that there are constraints over what is achievable at any given time and the common thread that I see in wildly successful companies is that they managed to jump on those turning points just before they are there, first mover advantage and all that gumph.