The world is full of perfectly good cantis, v-brakes, and rim brake wheels that sit unused since disc has taken over. Using what we've already produced for its actual full service life is pretty hard to beat for environmental impact in this and many other cases. Making metal things has a large environmental impact.
Consider also that you could procure some hydros that are being trashed or sitting unused because there's something wrong with them that's not considered monetarily viable to fix, like needing a seal overhaul with parts that aren't readily available, but you might be able to do some make-2-good-ones-from-4-bad-ones tricks on.
If you were comparing in a vacuum, i.e. you had to buy either system new, you cannot divorce that question from your projection of how many rims you think you're going to go through over the life of either system. From there you need to think about what your habits are in terms of wearing through rims from braking if you use rim brakes. But, one of the major considerations is also even if you go to discs, whether your rims will die by violence before the wear-saving effect of disc brakes would amount to anything anyway. For many mountain bikers the answer is yes and the rim wear point is basically moot, but maybe not if you're a casual rider (unlikely to damage a rim by impact) and/or heavy braker and/or ride in conditions that are particularly hard on rims used with rim brakes.
