Assumptions not necessarily true:
- that both groups are completely intersecting
- existing treatment is as easy (or cheap/fast/etc.) as treatment with MDMA
- that MDMA treatment is fully explored and mature so it's effectiveness is maximized
- improving PTSD symptoms == erased all traces
Now, I'm not one to believe any drug magically erases PTSD that easily. Nothing to date has proven to simply erase a mental illness (well, short of death.) Long term side effects I'm sure are being monitored, though you can also look at people who have used less pure forms recreationally for years to get some bounds. What long term 'costs' are you referring to though that are worse than living with untreatable PTSD?