I'm going to leave it at this:
There are religious arguments to be made for or against the recreational use of a number of substances. There are health arguments to be made for or against those same substances. Sometimes, they overlap, but for the most part, they are two distinct issues. Most of the arguments here are making an incoherent cholent out of the two.
From a medical perspective, consuming marijuana is safer than consuming cigarettes, alcohol, and most mood-altering substances. It is also less chemically addictive than cigarettes, alcohol, and most mood-altering substances. Like other things, the danger is greater when it is consumed by younger people whose brains have not fully developed. How safe it is for any individual will depend on that person. It CAN be used as a gateway drug, but it is not inherently a gateway drug.
From a religious perspective, it can be classified among a myriad of things which could possibly distract from Avodas Hashem. IMO, there is nothing about marijuana that makes it rank higher than most of those other things, many of which are accepted mainstays in our lives. Further, it is my opinion that a KK like this one, where anyone who uses marijuana is deemed an oisvarf, does much more harm than good to the respect our young people will have towards the Rabbonus in general. When something that is relatively minor gets this much attention and extreme overreaction, while greater dangers are ignored, people stop taking the signatories seriously. The lack of nuance is, unfortunately, not surprised and is already expected.
(As an aside, my personal opinion is that the lack of nuance in general in our communities is the source of most of our "crises.")