I'm confident Luke Hughes and Jonas Siegenthaler will be big cogs in the Devils future at LD, but beyond that there is some uncertainty.
Graves is in the last year of his contract. Kevin Bahl has yet to establish himself in the NHL, nor has he really separated himself from Okhotyuk. Personally, I think both players will be very solid NHL bottom-4 defensemen, but maybe not in New Jersey. A big reason for this is that Shakir Mukhamadullin has more upside than either of them.
It is always important to keep a strong prospect pipeline, to supplement your NHL roster in case of injury and also, if you're a contender, in the trade market. Did we think Fabian Zetterlund would be in a top 6 role this year? No, but Palat's injury has made Zetterlund quite valuable. Do we really think all 6 of our starting D will be healthy for the next 70 games? It's unlikely, so there's a good chance the Devils are going to need to deploy Bahl, or Okhotyuk, or both.
I've wasted my time on worse.