No, it’s not a good idea to use YouTube as a video file backup service.
That's because, if you keep creating private videos, and none (or most) of your videos are getting any plays, YouTube’s algorithm will figure out at some point that your channel adds no value to their viewers or to YouTube, that you’re abusing their terms of service, and your channel will get suspended, your account could get flagged, or worse, deleted.
Never use a third-party social platform for your backups. Not a smart move if you care about your content. Even if you’re fully compliant, you never know what could cause your account to get suspended or limited, and you could lose all of your content, private or not.
Better to use backup services like Google Drive, BackBlaze or Carbonite for creating backups. Also, buy an external hard-drive and back everything up locally as well.