Problem here i think is the development team itself. Like sure if it were up to nintendo as a company they'd be releasing characters each month and have people pay for them. But realistically i doubt Sakurai and the team want to be stuck developing smash for who knows how many years, specially considering Ultimate's development began right after Smash 4.Honestly Nintendo would be crazy to leave money sitting on the table like that, regardless of the first fighter pass being well received or not.
Which is exactly why I'm not certain that it will happen.
If the Switch ends up living a really long time, i could maybe see another batch happening. But honestly i think Sakurai saying the development of these fighters will go up to February 2020 right from the get go means they probably already decided to end it there.