You're forgetting a couple of things.
1. Not everyone will play Blockland on Steam (including myself)
2. RTB is more than just addon hosting and friend chatting, it has a colorset preview, addon-management, addon preferences, and I believer Super Admins/Admins are also part of RTB or is that default?
3. It will still remain the only way to download things in-game (unless Badspot adds a page that links to the workshop in game, similar to how GMod has it).