I don’t think Boeing ever originally thought they’d get to make the KC-46 as the A330 MRTT was already well established and won the contract on capability and cost. Then the pollies objected (U!S!A!) and won the overturn, and so they had to make it work. There’s nothing intrinsically wrong with the 767 as a base; it’s just a very old design. Add to that the quality control problems that the company brought on itself, and which came to a head with the 737-Max, and Boeing really doesn’t look good.
NG may be much, much smaller, but they’ve made some great (if incredibly expensive) aeroplanes over the postwar era.