I pretty much doubt anyone will target avast! for a very long time... Not to mention size of tha "carry" load that would be required to carry all old definitions for all antiviruses...
P2P/FTP/WEB sharing for old databases
All those viruses need just to chose correct filename to download and know where to put it. //Some viruses alredy did that b4.
Old version of programs is easy to find.
Lots of people have CD's with old or almost old software.=Simple to collect if need.
Search egines is one way for finding. So it is not neseccery to have own list of download path, just be able to sort results, scan pages for links. Internet have lot of different search engines, not just google.
Spywares collecting user information now to some database on the internet. Virus can be able to use same database for own use (make zombies).
Zombies= PC of users who cannot manage computer well and have low security and often connected to internet. How much are they?
Or... first wave: spywares for crypt-info collect
secound wave: viruses that uses same database.
Who knows how fast that can spread youself?
No one knows. I thinking that we are good protected and "all" have backedup computers and important documents.
Just restore them and continue to work.
...BTW internet become be faster and faster so you will not se that connection become be slower....