Is that what you tell a mom who states they plan to file for primary custody who has been an equal caregiver? That filing for residential custody is hostile so they should not?
Actually, yes, that IS what I would tell a Mom who offered up the exact same scenario.
Also, it's not in DAD's best interests to fight a battle "at any cost" if the climate in his local court is such that there's NO WAY 50/50 will be ordered if it's contested.
But, if dad files for primary custody, it's a hostile move?
Of Dad files for primary custody as a NEGOTIATING move (which is what you suggested), he's going to have to be prepared to fight ALL OUT, and regardless of cost to GET primary custody. Maybe Mom will settle for 50/50, but it's JUST as likely that they could end up in litigation for a year or so, spend 10s of thousands of dollars, and get a judge that NEVER awards 50/50 in a contested situation.
That advice? It's peni* neutral.