A new book addresses the fate of church buildings when congregations end:

Dominic Dutra, author of “Closing Costs,” a new book about how church property can be repurposed, says there are thousands of churches around the country that have closed or will likely close in the years to come. And too often, he said, leaders of those churches put off any discussion about what to do with their building until it’s too late.
“I’ve had situations where buildings are empty and they have no plan at all,” he said.
A 2021 study from Lifeway Research, based on data from three-dozen denominations, found that 4,500 churches closed in 2019, while only 3,000 were started. The 2021 Faith Communities Today study found that the median worship attendance for churches in the U.S. dropped from 137 people to 65 people over the past two decades.
Dutra argues that billions of dollars in church property could be put to work for ministry — if church leaders become proactive about the future. He has worked with a number of religious groups to do just that.
The numbers cited above are interesting: prior to COVID-19, more churches closed than opened. Additionally, the data from the survey is consistent with the National Congregations Study run over the last two decades regarding the median size of churches.
This is one area that my co-author Robert Brenneman and I did not address as much as we could have in our 2020 book Building Faith: A Sociology of Religious Structures. One of the later chapters looks at the fate of church buildings in the Chicago area. We found big differences across four denominations and a number of church buildings put to other uses. Church building are used in a variety of ways, including used by new congregations, converted into housing or commercial space, razed, and preserved.
Based on the description of the book in the article above, my guess is the recommendation is that church buildings no longer housing congregations can be put to other faith uses. There is certainly opportunity, ranging from serving new congregations to housing non-profits or parachurch organizations to being home to community centers.