If the
Magic and
Wonder are ever "disposed" of, it won't be via sinking them to create an artifical reef. As others have observed, that is only done for vessels that can be economically stripped of hazardous materials, which is why most artificial reefs are made from smaller ships (fishing or cargo under 15,000 tons), not huge passsenger ships.
The ultimate fate of the original
DCL ships is almost inevitably going to be scrapping. That will likely come long after they have left the DCL fleet and gone throuigh second-third-fourth careers as cruise vessels run by niche lines in less developed markets.
The sad thought here is scrapping is
never pretty and has become uglier, now that most of it is conducted in the Far East in countries with weak environmental and safety guidleines. Here are some examples of what that looks like in Alang, India: