That’s a good question, but unfortunately I haven’t found a way to change the default members Order By selection. Sorry!
Maybe you know how to make. User buy a paid membership and move to the first place in the list?
Moving users to the top of the directory involves sorting, and as you already know, it can be done, but unfortunately that sorting option cannot be made the default.
Another option could be to limit the directory to paid memberships, with:
[bps_directory role='name of role']
[bps_directory bp_member_type='name of member type']