When the news broke about all digital AM, I thought back to KYND's owner (now deceased). He was ready to jump on the bandwagon once it was approved, if the receiver interest existed. He understood the barriers and potential. That was in the 1990s or around 2000. As time moved on and things evolved, it became evident brokered time was the only viable option for an AM daytimer. That worked for many years. The station turned a profit. Then the FM translator for the AM signal came about. KYND applied and was shut down. Then a broadcaster in Dilley, Texas claimed to be retransmitting KYND 'off the air'. While KYND is a Houston station and Dilley is in the San Antonio market with KQQB in Stockdale on the same 1520 frequency as KYND, it got approved and a revised application for Houston was denied as the FCC said we already had a translator. To win the fight meant no frequencies available, so nothing to win, so to speak. So, KYND sat there: 25kw. days without a translator. The potential time brokered clients had figured out leasing a FM HD channel and leasing a FM translator. All that was left in the time brokered realm for AM were the poorly funded groups that could not pay anywhere close to what prior clients paid. The issue was more complex: more stations offering time than the universe of clients. This further diminished revenue potential. KYND got offers from potential buyers but none willing to pay the actual value of the land and equipment. Three years after the last client left, I did as well. See KYND's thread on the Houston page.
This is an actual case scenario. While a bit extreme, there are many AM stations in the same boat with a leak that will sink them, if not tomorrow or next week, next month or next year. The AM station, in many cases, could never afford the investment or would have the capital to give it a go. If this happened 20 years ago, it would be quite different, perhaps. I say perhaps because online options were already gaining traction by then.
As it is now, in some small markets, the AM loses a bit on money monthly but the FM pulls in a profit big enough to allow for the AM loss. If the small market FM does $25,000 a month and the AM does maybe $3,000 a month, you can keep the AM going. There's not the incentive to go digital.
About the only way a station might consider digital is if the company has a good number of profitable stations that can survive the possibility of digital AM going the route of FM HD signals.