I think there are pshatim that these people were the ones that carried out Nadav and Avihu (Mishael and Eltzofon) or that these were the people that were carrying Yosef’s aron. According to that you can say that their complaint was that by doing a mitzvah they lost out on another mitzvah. Eventually the result was that pesach sheni was created which allowed for many people to fulfill the mitzvah of korbon pesach in future generations-schar mitzvah miztvah.
My kashya is that the event of nadav and avihu was on 1st of Nissan which would have given them time to become tahor before the 14th of nissan.