hm... interesting...
i think a
bag designer are a person who designed the bag for a company or their own line (ex. joy gryson for her own line, karl for chanel, MJ for his own company and LV, etc).
AKA a mass production fashion company like zara, mango, topshop etc are not counted because the designer's are not "concealed" to the world. maybe they don't have a real designer, they only have an interpreter who interpret the latest trend from the runway and applied it to the brand fro mass production with affordable price.
but a
designer's bag are considered to be a term that's related to high end boutique's bag, it might also be a subtler term to say expensive bag. but because expensive is a "negative" word, so maybe they started to create the term "designer's bag" to replace "expensive bags"
that's IMO