As far as I can tell, the only way people are claiming they’re worse is by including “Modern Lubavitchers” in the calculation. I’ve never found it fair to exclude Ashkenazi MO from the Litvish world
There isn’t a clear demarcation between more or less modern yeshivish. Sure, the whole from world suffers from worsening adherence to proper dress and interaction of the genders, but it’s really quite noticeable in Chabad. At least in the tristate area, and I believe elsewhere as well.
Try to apply the standard to any other chassidus and you’ll see where your complaint about the fallacy of the tznius argument regarding Chabad fails.
Have you ever seen Belz, Satmar, Get, etc. groups behaving and dressing as you see in Chabad?