I agree that “designer” sunglasses seem to be more durable and last longer for me. I gave up on “cheap” sunglasses after my Tommy Hilfiger ones fell apart. Granted, they still lasted longer than the ones from Loft and Banana. My designer pairs are still going strong after several years. I predominantly have Chanels but my daily workhorse pair is from Tiffany, and I have a sporty pair from Oakley. My husband has a few Ray-bans which have held up quite nicely as well.
I’ve dropped my Tiffany pair and accidentally knocked them off my head before onto concrete/asphalt, and they’re still fine. I carry small bags so I never bring the hard carry case which I find too large and bulky. I usually put them in a soft drawstring bag before I toss them in my bag. I prefer the soft microfiber ones from Oakley that can double as a lens wipe
Another argument for designer sunglasses is the ability to repair/restore them. One of my Chanel sunglasses is white and had developed some areas of minor discoloration. I had purchased this pair about 10 years ago from a department store. Just earlier this year I brought it in to a Chanel boutique, and they sent my sunglasses in for repair - they were very kind and did not charge me because they believed it was due to a defect in the coating. My 10 year-old Chanel sunglasses now look as good as the day I purchased them!