If we COULD organise every single person to consume as little as possible (in terms of goods, fuel, electricity and services), that would mean that all those polluting companies have nobody left to produce stuff for. The 70% number doesn’t exist in a vacuum…

Good point. They would require an alternative supplier for the means of subsistence, however, which marks the limits of focussing on the consumer as opposed to the producer.

It’s just impossible to get enough people to stop buying stuff without a carbon tax and other rules that increase the cost of pollution.

That's the contradiction: you won't get the carbon tax until the masses organise to put pressure on legislators. Politicians aren't held back by a lack of public support (okay there are a few who would take action)—legislatures don't want to implement any carbon controls. They aren't guided by morality or abstract rationality.

First, this means, that one day they will appear to act spontaneously, morally, but this will be to avoid leaving stranded assets.

Second, they take actions that are logical in the context of class struggle. By this, I mean, there's a way of imposing a carbon tax without increasing prices: by taxing the energy companies. That won't happen in a bourgeois democracy without massive public pressure, because the politicians and energy execs tend to be members of the same class.