I can't prove it but it's my belief that MZ seasons were originally intended to provide an opportunity for hunters to use primitive weapons without having to compete with rifles and crowds. There is no question an inline is an easier entry point compared to traditional MZ. That has led to more hunters during MZ seasons, also no question. Whether that affects overall harvest numbers of animals would be a deep dive in harvest stats over time. Many things have changed since then and the primitive weapons aspect may not be true anymore. In Colorado at least, Its also probably true that much of the appeal of hunting with a MZ is the chance to hunt the rut for elk. Or the weather at that time of year. Hunting third and fourth season from a camp can be pretty burly experience.The way it could affect someone is for more people to start hunting with multiple weapons because inlines are more available. That could lead to more harvest and less opportunity for the other weapon types. That's especially true if you don't have to choose "one" weapon for the year.
Also, if more people are out during muzzleloader maybe more shoot that great buck someone was wanting to shoot during the firearms season.
But, there is nothing to prevent you from using a muzzleloader during firearm season, so it would work both ways. Why do you need to hunt during muzzleloader season or have a special season?
That being said, I don't care what you use as long as it's legal.