londrum - 8:32 pm on Nov 16, 2013 (gmt 0)
If Google consistently ranks a page number 1 for ten years (for its main keyword), then overnight that page falls to number 600, does it mean that for ten years that page was the best result on the web for that query, but now suddenly there are 599 better results?
best for who? imagine if ten people searched for a relatively simple phrase like "red widgets". what is the best for one person might not be the best for another. they could all be looking for slightly different things (prices? info? pictures? reviews?)
so it's possible that there are many "best" sites for each query. i think it makes sense to return different sites at no.1 (but dropping them 600 places would seem a bit over the top!)