It's the powers that be (money; pharmaceutical industry) who want these substances to remain criminalized, because these substances can be way more powerful and much cheaper than anti-depressants & Co. - and without destructive side-effects. So what MDMA, LSD, psilocybin and others can do is make people much healthier and the industry a tiny bit less rich, which is only in "the people's" interest.
>> You can mess up your serotonin levels for an extended period of time through MDMA abuse.
You can do that with SSRIs too. You can also kill yourself with opioids. The problem is that MDMA isn't even available by prescription. The first I heard of it was on a TV special covering its use in therapy sessions - that was in the 1980s prior to the whole ecstasy fad. Why is this useful substance totally illegal, while much more harmful and addictive things are legal, available, and causing problems?