Biggest problem with this idea is that users want the most relevant sites at the top. My guess is that you are thinking of commercial SERPs. With those, if widgets is a commonly sold item, all the top 20 sites for "buy widgets" could be seen as interchangeable. However, on info searches often they can be ranked best to worst, which Google with PageRank tends to do a decent job with. Also, on some topics there aren't 20 worthwhile sites. As an example, for some relatively obscure 1970s bubblegum rock band, there may be only one dedicated fan site about them. All the other sites have just passing mentions of the band. By rotating the top 20, that one dedicated fan site, which before always was at the top, now may end up in the #20 spot buried. This would lead to searcher dissatisfaction by making this site hard to find in the SERPs.