The tipping point for the majority of people to switch over to streaming radio seems to be years away so the big AM stations will stay on for as long as the owners can still make money with them.
Land under the towers of less viable stations will continue to be sold off causing gradual attrition.
Owners will also start selling some AMs to religious, ethnic, brokered and hobby broadcasters along with radical crazies who want a podium, even if it's on a dying radio band. Ultimately I believe this is what the AM band is destined to become and it will probably remain that way for decades, with many of those stations hanging on for a long time after the commercial viability of the band is long gone.