The rig was worth about $3 million as scrap in Turkey, according to the article ($190/ton at 17,000 tons). Approximately $0, maybe less, if the owners were forced to send it to an EU scrapyard, as regulators keep threatening to enforce.

At that price level, sinking it to create an artificial reef for environmental or tourism benefits is a competitive option. Though there may be other reasons it's not best suited for that.