Don’t sell them if you don’t want them to be used for whatever the new owners want to use them for.
Well, apparently the church feels financial constraints, what with declining numbers attending church, and probably related declining contributions.
You are right, if they decide to sell, they can’t have any say in what happens to the property.
I’ve heard that, many places have unique zoning for places of worship, which is part of why Islam takes over some old churches. Since the property is already zoned for religious use, Muslims find it easier to take over such property, rather than seek out brand new locations.