There's no way to chart the average review score over time, though, is there? If people saw several years of 4.5-averaged scores and then a sudden drop to 3-average, that would tip them off, but looking at dates next to individual reviews doesn't help because any single review by itself varies wildly with the knowledge and opinion of the reviewer.
As for immutable reviews- that's a good point, although it seems like you could get around that by leaving reviews editable through at least the listed warranty period of the item. Or add a separate set of reviews that only become active after a certain period of time after you left your first review, but now we're getting into anal-retentive nerd territory that no average user is incentivized to venture into. ("Why, of course I'd like to log back on to a review site I used once six years ago and talk about how my washing machine is functioning!")
Edit: Actually, the type of person who would seek out and help populate a "reliability rating site" might be the type of person who would respond with details to a six-year-later email that asks for commentary on a product, especially if you can enter the review straight from an email reply.
Interestingly they only allow for recommend / not recommend instead of stars.
In theory, that’s what the dates next to Amazon/App Store reviews are for. The problem with making them immutable though is: say you write a 5 star review, but the next day, the device craps out (inside the warranty). You contact the manufacturer who informs you that they can’t help you because it was caused by “user error” or whatever (basically, you can’t return/replace it). So you’d like to amend your review to mention that it crapped out, but you can’t. There also is the point of reviewing the product, not the company, but if a product and company sucks, why not warn the other consumers?