I didn't word this very well. What I mean is that there are/were no huge engineering challenges involved. We can't build a space elevator at any price, we can massively reduce our emissions by 'merely' spending a truly unfathomable amount of money
Do you have a source on it being such a large amount of money? My impression is that just like water rights we've priced the externality so low that a lot of things weren't done even though they were cheap. Here's a 2007 analysis putting the cost at less than 1% GDP:
What makes ditching fossil fuels on a civilization scale difficult is that ditching fossil fuels on an individual level is only easy for a portion of the population - a portion very much over represented on HN. If it were easy for all individuals to ditch fossil fuels (sorta the level of easy it is for all individuals to use paper instead of plastic straws) we'd pass some laws and ordinances and get it done in the blink of the eye. There would be push back sure - there are always reactionaries for everything - but the problem with EVs (and the problem that increased battery production scale might solve for us soon) is that it's expensive to switch to an EV and if America has spent more than a hundred years struggling to try and actually keep a roof[1] over everyone's head - getting a car in their pocket isn't going to be any easier.
1. I know it might feel the opposite at first glance when you think of how much EVs cost compared to a house in SF - but bear in mind that houses can be built extremely inexpensively.