That is just idealistic and will never work out. It is too hard to balance the two formats acceptably.
What is to be gained is to help the people wanting to raid a struggling format to actually raid it.
You seem to misunderstand me completely. It is not a new achievement. It is the same achievement with the tag changed depending on your kill.
While I support and enjoy the existence of two formats, 10- and 25-man should not be part of the same race due to the difficulties in balancing them. The amount of guilds that I have witnessed first hand going the smaller size simply to protect their ranking, and to ensure that their guild will live to the next tier, is huge. It should be possible for guilds to raid the format they want without having to ask themselves whether they should change in order to remain competitive - because the competition should be happening within the sizes. I would also dislike the current model even if the situation was the complete opposite.
You keep arguing that it is the choice of the community. That doesn't mean it is done without pressuring.
You also argue that Blizzard intends to keep on pretending that the two are identical. I will argue against them. To me, it is a bad decision and not the only one of its kind.