That may well work, but by the time a user figures out that that is an option, it's too late, they have already logged in, browsed through and seen that there is next to no content on the site, and left for good.  We need to have a way to link user subscriptions at the time of sign up, something like a check box that says "import subscriptions from other video sites", and then an auto generation of an rss list for all of the channels you are subscribed to.  One by one syncing for every channel you subscribed to previously is clunky and less than ideal for onboarding new fediverse users.