Its not irreversible if you are in good health. I think the only issue is when it becomes habitual or something you rely on to produce a certain state, instead of something that just introduces you to that state. I also think that it is important to be aware of how you are taking marijuana, as with any other herb or substance you are putting in your body. its quality, strain and how and by whom it was grown are all important factors as well as if you intend to smoke it, ingest it, vaporize it, tincture it, use as a salve, use as a soap, use as a fiber, use as a fodder.....
I think its a pity to be absolute about anything without considering the user, their mental, physical and emotional health and lifestyle, their intentions for use, the plant, its quality, how you take it, etcetcetc.
I do think that the way we use these substances in our society is definitely destructive, but that is just a product of our times. I mean, we use corn, soy and wheat to our own demise, so its no wonder we have absolutely no respect and no idea how to use these other plants in a beneficial way. I don't think that marijuana, opium, coca plants or mushrooms are intrinsically evil, we are the misusers and abuser, not the plants. they have lessons for us in that, and they have greater lessons for us when we respect them too. those lessons can be reaped by respectfully using them internally when they are the most appropriate medicine for mind, body or soul, or just respecting them externally and allowing them to flourish and heal environments and have their other beneficial uses like all plants do.