I always found this part of the story strange/Intresting.
From the start when the BC team took on the task of doing BC they wasnt even sure they could do it, in fact sounded like a lot of people internally didn't think it was possible.
So during the design of XO, they put in support (used silicon and design time and money) to support something, they must have had very little idea if it was worth it or not.
I've always justified it as being not that they are legacy support but probably widely supported formats that would be worth having regardless of BC, and it just happened to help BC.
So even without thinking of BC probably would still have them in, it just helped having more reasons to include it.
Always been my personal take.