With all due respect, I think you're making a big assumption by claiming that people who think this will cheapen their brand are caring what other people think, in the negative sense.
When a brand comes out with a lower end line, it "cheapens" the bag. Period. It's not the fact that anyone is looking at the bag and going "Oh there's a Botkier. It must be from Target! ::sticks up nose::" it's more about the fact that maybe someone would have gotten another brand from say Chloe or BV (who wouldn't do a Target line) because they wouldn't want their bag mimicked in any way (whether it's a knock off, a lower end line, etc.)
Sometimes we pick bags based on their individuality or even their exclusivity because it feels good to carry something one of a kind, or a designer a little "under the radar." I don't think those like myself are caring what other people think as much as understanding that having Target come out with a bag will make the designer VERY accessible - and let's face it - a lot of us like bags that are unaccessible from design houses who most have very little knowledge of because it's nice to carry something one of a kind and not emblazoned with monograms.
Personally, Botkier is my favorite designer and where I am from, they don't really exist, so I am a little disappointed that many people will be carrying around a bag that may not be the same quality as my bags but still sort of inspired by.