It's not that people are acting stupid and should know better, a lot of people are just incapable of understanding the implications of a complex situation like this.
Slashing energy usage means slashing many of the benefits of living in a post-industrial society. Think living in one-room houses (no cheap heating), eating bread and potatoes in winter (no imported food), working locally with little career progression (no easy commuting) and many people returning to manual labour (no cheap machinery). All this to prevent 60cm-1m of sea level rise by 2100 - something we can manage, if we maintain the civilisation we have worked so hard to build.
Your argument is equivalent to saying, in the 60s, "These guys believe the future is in toy computers that fit under your desk. If they succeed, the majority of people will have to turn to abacuses to do their taxes."
Your summary of my argument is completely off-the-mark.
A combination of insulation, thoughtful design (south-facing glass, adjustable awnings, solar water heating, heat pumps vs. resistive electrical heat, etc.) and renewably sourced electricity go a long ways towards the heating issue. I live in a cold-ish city in a 2 bed apt with only electric storage heaters (which are awful) and pay for supposedly 100% renewable (wind/solar/geothermal) energy and my bill peaks at €65 a month in the winter.
Commuting:
There are also thousands of businesses within a 30 minute walk from my home. Not turning cities into parking lots and skidpads with a building dropped here and there helps. If I cycle, the number of businesses in that range increases by about a factor of 9. (I cycle around 3 times as fast as I walk, so can cover a distance roughly three times as far, or area ~9 times as great). If I don't like pedaling, an e-bike or 50-150cc scooter/motorcycle will get me even farther.
Food:
I'm not well informed enough on this to comment. Where I live already has pretty bland food with loads of bread and potatoes. Good burritos, though. I'd miss that. We'd probably eat a lot less meat. I could do with a better supply of soyrizo, anyway.
Effects:
Sea level rise is far from the only concern. Stronger storms, drought, the displacement of peoples and an ensuing refugee crisis as their crops fail and water supplies dwindle, ocean acidification, and changing weather systems from the loss of the Arctic ice cap (and possibly a chunk of Greenland) are just a few of them. Also, I suppose this makes me impossibly impractical but hey, the loss of polar bears would make me sad. Maintaining the civilisation we built at the cost of the natural world we inherited isn't fantastic; watching the forests of my home (California) be completely devastated by drought, heat, and beetles is a bummer, to put it mildly.
As for the effects of climate change, they look intimidating as a whole - when you look at each item individually, they are unpleasant but manageable. Humans have been dealing with the weather for millennia, after all.
> Stronger storms, drought
These happen routinely now. Yes, it would be preferable to have less storms and drought, but we know how to handle them.
> the displacement of peoples
In the last decade the Chinese have built scores of new cities across the third world - it's not that hard for people to move.
> an ensuing refugee crisis as their crops fail and water supplies dwindle
We grow crops in many different climates now. Crop failures happen now in poor countries with bad infrastructure and management. Societies with sufficient capital and technology can adapt to almost any climate. (In extreme climates, with desalination plants and hydroponics - or people could just move).
> Maintaining the civilisation we built at the cost of the natural world we inherited isn't fantastic
This is really the crux of the issue - should the standard of value be maximising human flourishing, or minimising human impact?
If we took the very dramatic changes of switching all energy to nuclear, changed to a mostly vegetarian diet, and replaced as much of transportation usage as possible with electric we would end up with an 80% or greater reduction in emissions and still enjoy plenty of the benefits of a post-industrial society.
The solution to climate change is not simple. Yes, emit less CO2 and other greenhouse gasses. But the amount of things you can supposedly do to achieve that are numerous and the effects are maybe not even visible in your lifetime.