It's very much tragedy of the commons, but say with the classical example of wells on a combined finite water source, using less water won't make it last longer when you represent a permille of the total consumption, and you are only really hurting yourself by not using it while it still exists. Meanwhile the bottling plant next door pumps half of it for free and sells to back to you with a label that says "save water".

A single vote can change nothing or change everything depending on the overall distribution. If the public opinion is 80% towards one candidate and 20% for the other, even 30% of the votes wouldn't matter. If it's extremely polarized like 50/50 then every vote does indeed make a difference (Gore vs. Bush, topically enough lol). The problem with climate change is that it's more like 95% of people doing nothing cause they just want to live a normal life, and 5% of people trying to do something in a futile way. In the end it doesn't matter if a few million choose to cut down on consumption when the end result is exactly the same in an ocean of billions.

> how do you get a systemic change? By influencing individual voting and consumption behavior.

That's more like a democratic fantasy, believing that most people would even chose to to the right thing if given the option, instead of the most convenient thing.

No, you get it by lobbying key people who have the power to enact change on a massive scale. As you say, corporations only care about legal means and they make sure those means stay in their favor.

One might argue that if enough people can be persuaded to stop using a product (e.g. gasoline) then the production of it will lower, but in lots of cases that's a fallacy as well, since the producer will move against an existential threat to itself with all the means at their disposal, PR, legal, or even less legal. And in a sufficiently large market, any impact a single person can make is again a drop in the ocean.