I say orthoprax, because many of these people will openly act as if they are practicing while at home or in their community but will go off to do other stuff when alone. So strictly speaking you can take issue with the term orthoprax if you want. Would you prefer the term ‘sometimes pretends to be frum, but willingly violates kashrus and other mitzvos when unobserved?’
I can think of one particular man I knew who was a member of a Chassishe family and stayed a respected person in his community, but would sneak off to very non-kosher strip clubs from time to time.
I agree that some people will leave orthodoxy and don’t go anywhere else, but those I’ve heard of probably wouldn’t identify as orthodox anymore. More likely as unaffiliated/none.