Earlier quoted context omitted.
I think they could do like ebay does: reviews in the last 4 weeks, 3 months, 6 months, ...etc. Or a similar mechanism. Sometimes I am not interested to old reviews, esp. technical books, as they become very quickly outdated. Deleting them implies deleted forever, though. Such information will be always lost, while it might still apply. I would simply give it a different weight.
Aren't those time limits on reviews only for reviews of sellers , though? Does Ebay also time-limit products reviews, reviews that are divorced from a seller? Do they even host those kind of reviews?
However, I am quite sure I have seen elsewhere this feature. It's not bad. It allows to have more recent opinions about the item.