Technically, your account is on the game it belongs because it's on the game you've played. No harm in starting over though.
I'd suggest reading a bit about the release of OSRS to get a better understanding of the circumstances:
This wasn't like the transfer of RuneScape Classic to RuneScape 2 in 2004 where players were given an option to transfer over or not. Old School was released in 2013 as a brand new game from a back up of the game from 2007. Transferring an account to a version of the game that is 6 years older wouldn't have been feasible. And, doing so would have been detrimental for the integrity of the game.
After the Evolution of Combat (2012) and New Interface System (2013) updates, RuneScape 2 was rebranded as RuneScape 3 as a marketing ploy. However, it is still referred to as just RuneScape, and it's running on the same game that began with RuneScape 2's release in 2004.