So what do you do when a partnership dries up unexpectedly? Or is this one of those situations where dance partners should be like computers -- always have a backup?
too often thats the problem

I mean the partner has a backup that you did not know about.
you can't reasonably have a backup. An AM dance partnership is more committed than a marriage; a backup would cause an instant divorce - and lots of crying.
the answer is you stop dancing until you can face the humility that the dance is over - and then you either quit (since there are no other partners) or if you HAVE to dance you do pro-am.
guess where Iam...

[but actually loving it

]