Musketeers are easily as good as Janissaries. If you think muskets have a short lifespan, Musketeers at least have the luxury of being able to get there to cause some trouble for your enemy. And it can mean big trouble for him of you do this right.
Janissaries are great on paper, and if you are more advanced, you can pretty much move about unopposed for a while. But they do have a counter - muskets. The enemy might start building muskets of his own soon, and then you can no longer consider your Janisaries invincible. And they don't have the mobility to always contribute very much before this happens.
Come to think of it, isn't the Musketeer the best possible counter to the Janissary in the advent of gunpowder?