It makes sense Edna feels strongly about the capes. She designed those outfits, so maybe there's a little guilt in those heroes' fates. What's undeniable is that she learned a lot since then: sure, you need to look dapper and flashy, but being able to do your job without your freaking outfit getting in the way is the most important part of your suit. The fact that she can also make them frictionless, able to become invisible, practically indestructible, fresh to wear despite being made of what looks like very tight spandex, AND can be washed in a washing machine... That's just the icing on the cake.
I mean it wouldn't be hard to design a way for the cape to detach if for example someone pulled on it. I used to work for a security company that had ties that literally detached from the neck if someone tried to pull on it. 😅
I always had a theory that Edna designed Syndrome’s costume at his request claiming to be a new superhero, but she was able to catch on to the fact he was up to no good and deliberately sabotaged his costume by including a cape in the design