You don't but Seattle's defense with Michael Bennett is a lot better without him.
Will we be fine without Hardy? Probably. Our front seven is extremely talented and I like the upside in our secondary. Our defense is even better with Hardy, though.
We don't need him nor do we need to throw buckets of cash at him. I wouldn't be in support of that. However, if we could bring him back on a cheap deal, I don't see why we wouldn't look into it. Hardy makes our team better.
NOTE - This is not me saying Hardy will be back even if we were open to returning on a cheap deal. I'm not going to get into a discussion about that. What I am saying is that our defense is much improved with Hardy on the field. He is a top flight pass rusher.