If Mapillary should go bankrupt or just change the rules for download, it does not matter. The only way authorities can secure their data is to do it themselfs. I.e. have a local copy, an offsite copy and test a restore regulary. Preferable more copies, because when one copy disappears it is pretty fragile to only have one left. Disasters are flock animals and often comes in many folds.
Mapillary do offer business deals, and part of a such could be an easy way to get their original data. That would count as the offsite copy.