Avoidance only reinforces PTSD. It's literally the opposite of what you should be doing to treat it.
Trigger warnings are designed to help survivors avoid reminders of their trauma, thereby preventing emotional discomfort. Yet avoidance reinforces PTSD. Conversely, systematic exposure to triggers and the memories they provoke is the most effective means of overcoming the disorder. According to a rigorous analysis by the Institute of Medicine, exposure therapy is the most efficacious treatment for PTSD, especially in civilians who have suffered trauma such as sexual assault. For example, prolonged exposure therapy, the cognitive behavioral treatment pioneered by clinical psychologists Edna B. Foa and Barbara O. Rothbaum, entails having clients close their eyes and recount their trauma in the first-person present tense. After repeated imaginal relivings, most clients experience significant reductions in PTSD symptoms, as traumatic memories lose their capacity to cause emotional distress. Working with their therapists, clients devise a hierarchy of progressively more challenging trigger situations that they may confront in everyday life. By practicing confronting these triggers, clients learn that fear subsides, enabling them to reclaim their lives and conquer PTSD.
In a controlled therapeutic setting over a period of time, desensitization can be very effective. But it's a not a magic switch - it doesn't work right away, doesn't always work completely, and doesn't work for everyone. So trigger warnings are still a good thing. Not everyone with PTSD knows it (it took me a surprising amount of time to figure out what was causing my panic attacks), can get into therapy, or does well with therapy.
People with asthma often have a maintenance medication and an emergency inhaler. People with allergies have daily medications and epi-pens. People with PTSD need therapy and also sometimes trigger warnings. People are fine with the first 2 but not with the third for reasons that often boil down to 'it's inconvenient and also I don't like it', which isn't a reason to knock down people who are moving too slowly, succumb to road rage or drop the f-bomb at a family dinner but apparently is a good enough reason to cause panic attacks.