I have an amateur website up and running where club members can book an accomodation.

Most other software wants more than the $100 one time payment that some random WordPress plugin demanded (VikBooking.)

A lot of people use Wordpress for multi-user or business sites and there isn't a replacement that will meet their needs that also doesn't require a higher degree of technical knowledge than they have. Any alternative more complex than Wordpress's install or that doesn't have feature parity with Wordpress' plugins is going to be a no-go for most of them.