I wouldn't exactly say Steam workshop is secure for file storage. Who's to say in a decades time, Steam decides to discontinue the SteamCommunity site - taking along most of the user generated content with it? That would be a bad thing to see happen.
Theoretically speaking.... What if we created something inside the game client to pull in, sync, upload user generated content?
I wouldent mind if the game is not on steam but i think you would have more sales if you get the game on steam.
Why not have both options? Buy without steam or with steam.
If there was no steam i wouldent know this game existed, i discovered this game on the steam forum.
Good luck with the game.