Considering that said microwave uses a lot more electronics than said oven and lasts 1/3 to 1/5 as long as one (per your own observations), between the energy consumption (because you are almost certainly not getting your energy from renewable sources) and the increasingly large bill of materials (with the microwave requiring many more exotics) averaged out over total lifespan, I would bet that a microwave and a gas oven are quite competitive over the long-term.

But the microwave is producing more waste as it isn't built to last as long, and the items you cook in it are probably wrapped in chemical-seeping plastic and printed cardboard. So the volume of waste with use is considerably higher as well.

And I'm not even considering electric ovens, which are becoming increasingly mandated.