what i mean is the following.
Earlier tukish army was traditionally cavalry. and it was an excellent one.
Since the othmans were smart badasses, they understood that they need a solid infantry, too. You can sweep the hungarian plains with horsemen (or the anatolian liplane), but if the terrain is broke, you're toast.
thus the "new militia" (that's what Jannissery mean) was formed.
And the way it was formed made it loyal to the sultan (cavalry was often more skiddish and unthrustworthy, more loyal to local lords).
That's a good thing, having loyal soldiers around. thus most of Costantinople garrison was made of them.
And that caused the same situation the roman emperors had with their pretorian guards, which basically prevented any reform of the corp other than "rising the pay".
Now the Jannissaries themselves did not start as a bad infantry (e.g. they had an high espirit de corp), but they had the limitations of a society that traditionally did not have an infantry of sort and tried to improvise one. That would have not been a critical problem it the corp had gradually evolved towards better arrangements, but their status of half-guards, half-kidnappers of the sultan made them able to stem any attempt of that.