These kinds of things don't take place in a vacuum. The decision on whether or not to re-allocate the AM band to some other purpose will be made by politicians and bureaucrats, not by the business people who operate radio stations. If a small number of AM stations remain profitable but the majority are losing money, and if other businesses are demanding access to the frequency band allocated to AM radio, it will come down to a question of which group is petitioning the government for action more effectively. It's not a question of media companies making a unilateral decision. It will come down to the companies who are operating the last few profitable AM stations deciding which they'd rather have, continued access to the AM band or some other profitable thing.
For example, as a purely hypothetical example, if given a choice between the government expanding the current FM band by allocating more of the current spectrum so that there would be more FM stations combined with a relaxation of the rules so that those few operators of AM stations could have extra FM stations versus maintaining the status quo on AM, how many of the corporations operating AM stations would chose to support maintaining the status quo? Bear in mind, those corporations would also probably stand to make nice profits from their endeavours that would benefit from alternate uses for the current AM band as well.
The example eriedj posted, WBBM-AM, is a CBS owned and operated property. The content of WBBM-AM is simulcast on WCFS-FM. Do you think CBS would object to losing access to 780 KHz in an AM band reallocation if the government would allow CBS to own a few more TV stations, or some other equally valuable consideration?