I think more appropriate would be for older ratings to not be used in the calculations after a length of time (5yrs, 10yrs?), and only be included after new ratings.
AcDc (IMHO) was as close to a 10 as was possible when it first came out, now however there are games that have surpassed that in terms of code, sound, display, etc. (again IMHO)
How many people go back and lower their ratings of games they rated 5yrs ago to reflect their standing against all games now available?
MB, AFM, TAF, TZ etc. can't stand up as well against as they do if they were rated now.