I'm wondering why you're having it as a stated replacement for the Nodachi and Hayabusa II, though.
Having been waiting for this question, the explanation is hopefully as simple as it is in my head.
The Mamushi is only a potential replacement, which probably won't be actually designated as a replacement for a while. The majority of the possible replacement idea comes from the idea of unified airframes. It is easier on IC logistics to have carrier-borne aircraft have as much use as they can squeeze into them as possible due to limited numbers of fighters, hence the Multirole name. As a Multirole, it is designed to be able to do just about any mission by being able to mount nearly anything. While one fighter may be designated as a superiority fighter and fitted with more high-maneuvering torpedoes, another could be set up for atmospheric close air support by attaching two mini-missile pods and bombs to the external pylons while keeping two to four torpedoes in the interior hardpoints for Air-to-Air/Space-to-Space/Air-to-Ground combat.
While yes, a single Sharie-Class Battleship carries a full wing of sixty Fighters, some ships may only be able to carry ten fighters total. Instead of trying to change up and scramble pilots on different starfighters and different strategies for different airframes, the Mamushi was designed to do the jobs of both in a single airframe.
For a real world reference, it is like the F-14 series and the F/A-18. While designed to supplement the other, one airframe ended up replacing the other due to logistics, ease of maintenance, and newer version tech.
I am not trashing the Nodachi or the Hayabusa II. They both have good art and good styles. The purpose of me putting the potential replacement sentence is the option of being able to select a new generation starfighter as a primary option in, for example, five years or more, when the Nodachi and Hayabusa IIs are much older. If it replaces anything, it will probably replace the Hayabusa II first before the Nodachi, due to the base missions specs for the Mamushi to be considered a newer generation than the Hayabusa II.
TL;DR: They will be serving alongside each other for a while anyway. The Mamushi will probably replace the Hayabusa II faster than it would the Nodachi but ever still this would happen a fair number of years later. The Mamushi has the ability to do the jobs of both the Hayabusa II and the Nodachi, but not at the same time.