The Archivers systems above save them if you submit it to be saved though their API. All this is saved outside SC infrastructure so no need to worry about size and storage.
Backups and and github are not good at preserving the site snapshot and citations. If the site does go down the the content will still be inaccessible until it is brough online again even from a backup. What if the site maintenance and outage is aggravated by internal issues like: Ajahn Brahm Has Resigned From the BSWA. One reason SO / SE backs up a snapshot to 3rd party archives sites is a hedge against organisational issues. What if at time of outage SC has lost it’s volunteers to bring back the site up again. What if 3rd party can’t put the needed effects to bring it up again from Github. What if you come to a point here is not enough donations to keep the infrastructure running. There are lot of ifs here but these are all possibilities. No would have expected USSR to collapse in the 1980s. There are a lot of active sites and projects which suddenly go dormant. An average Internet will not be able to access the backups or use github to bring the site up again, but will readily available to browse the Wayback Machine and Archive.today snapshots.
Though general discussion may not merit archiving, resources (attachments, URLs, etc) do merit being archived.
Also it might be a good idea to mirror Github repo in Gitlab also. It wasn’t long ago that BerliOS and Codehaus were the most active code hosting sites.
So while the project is at its peak, invest in preservation also as a contingency in case anything unforeseeable happens. This may not pay dividends in this generation would be useful in generations to come provided the archive sites also survive.