Think about it. Within 2 weeks, the black mesa source trailer had 500000 views. Now say if everyone that watched the trailer watched it twice, thats 250000 people.
It now has over 1 million views. Say that every one that watched it by this point watched it three times, thats ~350000 people.
So far, Three Hundred and Fifty Thousand people know the mod exists and are probably pumped for it.
Given that not all these people are dedicated fans, we should see approx 10-20 thousand people download the mod on day one, and that number will grow exponentially.
Given that the mod itself is a much bigger gamer magnet than a mere (but awesome) trailer, we could see downloads in excess of 500000 individual IP adresses by Release date +6 months
I expect ‘everyone’ that has watched the trailer to have downloaded it within 3-4 months post release, and then add in the latecomers for the extra 150000.
This is in my opinion a conservative estimate, as it doesn’t take into account further promotion my Valve if they happen to put it on steam and in their news feed.
If this happens, double everything.