Magnetrons are dirt cheap 2.45GHz RF sources. Unstable, poorly regulated, but dirt cheap.
You can improve them to some degree with power control, which is what inverter microwaves do. These can vary the power between about 40% and 100%, at less than 40% I guess the magnetron isn't stable so they cycle the power instead (maybe there's another reason but my Panasonic inverter microwave does this.)
RF power electronics would need to fall towards the $25 or so a microwave oven transformer, magnetron and capacitor achieve to go truly mass market, and at least be within the $100-$200 cost region to be at least in the premium market (say for a $500 oven.) It's a big ask, it may well happen eventually but it will take some time if it does.