That would tell 'you' when the last one was, but do you really want to add this type of maintenance to your 'to do' list? And if (accidents do happen

) the wrong one is deleted, there will be some explaining to do, to the Client -- since FP will no longer work for them.
Much easier IMHO, to have the Client be aware of what FP is doing in this regard, and pass the maintenance over to them. If the situation seems to get out of control (many, many, FP .htaccess files being created) you can always make maintaining them 'an account requirement' and see if that helps.
Just a thought.