First of all, who needs a museum dedicated to costumes, something of very little cultural value, IMO.
What makes costumes have less value than paintings, for example?
Could we have theater performances, musicals, movies, ballet, opera, figure skating without costumes? Sure. At the same time, I think costumes help make the performance and create the illusion, and not just for the audience. It feels different to step on stage or perform in a costume than in every day clothes; a costume is part of the character, part of the performance. It's part of the art.
Costumes also evolved through the years. I recently saw a short British video clip on YouTube where they had a look at some of Margot Fonteyn's costumes and I thought it was interesting to learn about why they used what fabric and color and what the effect was.
Before that, there was the ballet dancer who shortened her skirt to above ankle length and caused quite a stir with it because women's dresses were supposed to cover their ankles (even if her name now escapes me and I'm too lazy to look it up).
And before that, men were dancing female roles which would have been reflected in costumes as well since they're built differently.
My granddad's partner was a costume designer for the Friedrichstadtpalast which was
the theater in East Berlin. And yet, they didn't have big budgets or access to the same fabrics and accessories the west had access, too. So, they had to improvise and find alternative ways to create the costumes the Friedrichstadtpalast was and is known for.
That's just a small area of costumes.
I think there's a lost of history in costumes and based on the above, if I had to make a comparison and determination about cultural value, I'd say costumes have more cultural value than paintings. Considering how many museums dedicated to paintings there are, why not have a museum for costumes? (For the record, I think paintings have their place in culture, too. All art forms have, in my opinion; they all contribute in their own unique way).