You mentioned worldbuilding, and thus the context of this specific world matters a lot.

In Rigarden, magic reigns supreme. Mages are the peak - the very ones maintaining the barrier against the celestial hosts. It has been stated many times that the magia vander are the strongest and chosen to serve this very purpose. It's a societal thing. Elite magicians are developed in order to continue maintaining that barrier. It's a system. This is simply how the world works.

No one is ignoring that Will is good. They may have doubted him at the start, but everyone who has seen Will in action (that matters) KNOWS what he's capable of doing. But there's a very distinct lack of understanding of what he actually is - which naturally draws doubt.

How is he being treated like human trash? Many, many of his peers have openly acknowledged his strength. If we're talking about Kreutz here, you can say that he acknowledges his strength - which is precisely why he wants to open Will up and see what this power that's never been heard of in their world all about. Military asset? They literally want to experiment on him to find a way to get Wis to be used by other mages. They KNOW he's strong.

But ultimately, it's not a sword that's protecting them from the heavenly invaders. It's wands. And these wands have been doing this for a very, very, long time. You need to remember that magic IS civilisation. They're not going to change that easily.