I designed a system where you could say "donate this spare 2 TB of my disk space to the Internet Archive" and the IA would push 2 TB of data to you. This system also has the property that it can be reconstructed if the IA (or whatever provider) goes away.
Unfortunately, when I talked to a few archival teams (including the IA) about whether they'd be interested in using it, I either got no response or a negative one.
Because the incentive will be archiving things they believe should be archived, you need the process to begin with what urls do you want to be archiving, then people will be incentivized for archiving the juicy stuff IA is used for and you just throw some stuff they didn't ask to archive in the remit of them storing what they want.
You're essentially saying that having an extra copy of data is equally as reliable as not having an extra copy of data. I would encourage you to think about this a bit more.
Realistically you won't get enough volunteer-storage to cover one IA. And even if you did, it wouldn't satisfy the mission requirements, which is to store reliably for decades all of the data.
Yes, the idea is that this is a replacement for the torrents they make public. In case the IA goes away, we'll have this distributed dataset to fall back on.
Unfortunately, when I talked to a few archival teams (including the IA) about whether they'd be interested in using it, I either got no response or a negative one.