Supplies are currently used for literally everything aside from travelling in Hyperspace, even crew upkeep. Adding a new item for the crew entirely will not only make the game more immersive but also get rid of the constant supply drain.
How will splitting the current supplies resource into a 'food, water, and air' resource and an 'ammunition and spare parts' resource remove the supply drain? The crew will still be consuming the 'food, water, and air' resource at some rate dependent on the number of people present, and presumably there's some amount of maintenance, and perhaps gunnery practice, being performed on all the ships in the fleet, which would result in some consumption rate for the 'ammunition and spare parts' resource. All that splitting supplies up does is to create two resources which are consumed at some rate all the time, which you then need to balance when preparing your fleet.
Is it a potentially more interesting situation than having a unified resource? Yes, if you happen to like the logistical side of things. Is it harder to get right when you're new to the game? Yep. Does it get rid of the slow drain on supplies, be they the current version or the split version? Not for any reason that I can see.
It's also at least one more resource which must be displayed on the UI - both 'food, water, and air' and 'ammunition and spare parts' are things which are essential for any theoretical starfaring vessel. A 'food, water, and air' resource simply has to be constantly consumed by the crew, and while you could probably argue that the 'ammunition' component of an 'ammunition and spare parts' resource would only be consumed in battles (though I know of no real-world navy which never engages in some form of gunnery practice), routine maintenance and occasional component failures would impose a small, continual drain on the 'spare parts' side. Since (presumably) the crew will die without the 'food, water, and air' resource, and the ships cannot be maintained in an operational condition without the 'spare parts' side of the 'ammunition and spare parts' resource and cannot fight without the 'ammunition' side (or at least the weapon systems which use ammunition are unusable without it), both resources are sufficiently important to be displayed somewhere in the UI - presumably, it's a loss condition if your crew starves to death, and it's probably also one if your ships are in such poor repair that they can neither fly nor fight; it's certainly a loss condition if your ships are in such poor repair that systems are failing and killing off crew members or destroying ships.
Yes, it's currently true that supplies are the only consumable resource that you need to worry about when determining how you will allocate your cargo space. However, since the game is intended to include a trading component, we will presumably gain at least one resource which will compete with supplies for cargo space, and I don't see that balancing 'food, water, and air', 'ammunition and spare parts', and whatever we get for trade goods will be particularly more interesting than balancing the current supplies against whatever we get for trade goods, especially if we get multiple kinds of trade goods which have varied values dependent upon where you are. Another thing to consider is that fuel will presumably become more valuable as a resource once there's a good reason to make frequent trips between star systems, and between 'trade goods', 'supplies', and 'fuel' you do have a three-resource model - fuel for (long-range) movement, supplies for keeping your ships workable, and trade goods for acquiring the supplies to keep ships operational, the fuel to keep your ships moving, and the money to acquire additional or better ships.
I will agree that fuel could potentially be more interesting if there were something to compete with it, but on the other hand, if the fuel is a liquid or a compressed gas, it's possible to store useful amounts of it in areas of a ship which cannot reasonably be used for storing solid goods, and it's not at all unbelievable for the fuel to be of a type which makes storing goods in empty portions of fuel tanks, or in empty fuel tanks, undesirable. With liquid or gas fuels, it's also entirely reasonable for the interior of the tank to be inaccessible (or nearly so) without damaging the tank, and for any access points added to the setup for maintenance purposes to be less than ideal for cargo handling.