"who would have thought 50 years ago that we‘d need barriers againt car attacks at all major public events"
To be honest, we still don't need them. Obviously it's horrible but considering that we accept ~1.25 million people a year killed on the roads (~25k in the EU, 30-40k in the US, or about one 9-eleven a month) it's probably not the best ROI. Though to be honest I think we should put massive barriers between cars and people anywhere we can, or just remove the cars wherever possible, but I'm odd that way.
Like I said I hope I'm wrong, but right now it looks like all the earlier estimates were too conservative and to think that we can cause major disruptions to the world's food supply, as well as making some places simply uninhabitable without air conditioning, strikes me as naive. To think that you can do it while still planning a 401(k) and buying holiday houses on the beach a cheap flight away for when you retire seems especially naive (though I worry we'll still have the cheap flights, which would be a bummer unless they're electric).
As far as population - it's true that we could stand to have far fewer people around, but even if your motivations are purely selfish, making hundreds of millions of people hungry, thirsty, and desperate doesn't sound like a good idea. And of course, most poor people emit basically nothing, so reducing the population of people living on under 5 dollars a day won't do much for the planet as a whole.