The public liked K cars. They were decent sized, reliable and cheap. They sold over 200,000 Aries and Reliants a year until they were discontinued and probably another 100,000 on top of that if you add in the other models. They weren't all dead by 1989 but the major ones were because they were replaced with the Dynasty and New Yorker which were derived from them. I'm no fan of K cars or any of the other 80s Chryslers other then our cars but they did the job for Chrysler that they were supposed to do and they did it well enough to keep the company going.
I don't know if it would have been a good decision to keep M bodies going longer then they did but they should have at least invested more in them and given them a chance. They were supposed to be gone by the mid 80s and they only kept going because the sales were better then expected. But instead of just dragging the same car on year after year they could have at least done some improvements to it. GM and Ford had overdrive transmissions and fuel injection and that was already in Dodge trucks. They could have added it to the cars and that probably would have bumped sales and kept them going for a couple more years. That's probably why sales dropped at the end.