This would be good advice if there were a unitary "we" who had already agreed to pursue the least-cost route to climate stabilization. But Californians can't force Wyomingites to cut their emissions, the EU can't force Americans to cut their emissions, and so on. Human emissions are going to continue beyond the point where feedback loops can be damped simply by cutting back on combustion. When the warning is "only 12 more years" to finish a decarbonization process that should have started 30 years ago, you know that the problem won't be solved optimally.

It's tragic that everyone is going to pay more because humans couldn't coordinate soon enough to just do the smart thing, but that's how it is. The IPCC is discussing negative emissions strategies not because they're cheaper than cutting emissions but because humans are not implementing deep emissions cuts fast enough.