Avast doesn't retain old version downloads, but there are many sites that do, filehippo.com and majorgeeks.com are two that do and are reputable sites.
You are loosing sight of the fact that this is a support site and generally it is only those with problems that arrive at the forums.
You weren't around after the release of avast 5.0 almost a year ago, there was a spike in traffic on the forums, which is understandable with such a large (130 million plus) and diverse user-base with a myriad of different OSes, hardware and software, etc. Those problems were also resolved in a fairly short time as no doubt will these.