Today, I learned that local AM flamethrower KCBS added a simulcast on FM 106.9. In the San Francisco bay area, FM 106.9 has changed hands and formats a few times in the past decade.
The all-news KCBS recently carried unusual promos for FM 106.9, which previously used the callsign KFRC. My best recollection of those ads suggested that KFRC was playing oldies music, with some KCBS news. Hearing KCBS identify itself today as AM 740 and FM 106.9 therefore wasn't a big surprise, just a little one.
What's the case for simulcasting an all-news broadcast on AM and FM? I suppose the motivation is to expand listener reach, and therefore, advertising dollars. Younger people are probably more likely to listen to FM radio, and to scan through stations when listening to the radio in a car. They're probably scanning for music stations, though.
I rely on KCBS for traffic reports. The traffic reports on KCBS happen every ten minutes ("on the eights", as some other stations might announce). (Actually, I could do a whole post about the traffic reports on KCBS.) So, car radios without AM couldn't previously tune in to KCBS to hear about the traffic, but now it's available on FM.
Technology is progressing, and soon, most devices with a screen will be capable of receiving and displaying maps, driving directions, and traffic information.
Without knowing more about the plans, I doubt that KCBS will move off AM. As previously mentioned, KCBS is one of the stations in my area broadcasting in AM HD. Wikipedia calls KCBS "a leading contender for the title of oldest station in the United States and possibly the world." So it's just hard to imagine that KCBS would give up on AM.