If this does not happen in the next 15 years in developed countries then we have a giant problem. I understand that installing a gas heater right now might be a reasonable personal decision, but I don't understand how one could argue it's the right thing to do from a climate protection angle.

It's the right thing to do from a climate protection angle if a significant proportion of your electricity is generated by natural gas, which is what I said to begin with.

It is, but only temporarily. Will you switch it over to a heat pump immediately after the cut off point is reached or will you operate till the end of its life before you replace it? If you keep operating it, will it really emit less over its lifetime than switching to a heat pump right now? Making the assumption that today's electricity mix is a steady state is flawed.