I don't really view Mercer as a core component. Nice player, and there's some value in terms of play / contract, though I'm not sure how much because I'm not sure what the expectation of his contract is. But Mercer isn't a guy I'm afraid of losing either. He's a nice player. His role is also not the hardest to replace if we are being honest.
If they are okay with signing Saros (I don't think the Devils analytics department would be and I don't think devils fans love that idea either) to a long term deal, a Mercer / Saros deal just isn't that big of a deal for me. Saros could certainly tank after acquisition, or the Mercer that showed up this year could be closer to what he is moving forward. We just don't have a way to tell the future in that regard. But I'm okay with taking that risk. But again, its Saros' contract that is the issue, most likely.
The #10 pick could certainly have incredible value. It could also be Michael Rassmussen. It's a magic bean. We all know what's what with that. Could be great, could be terrible. Could be a whatever. It's a high variance resource which has an enticement that varies person to person based on various factors. If you value that more than Saros, that's fine. I definitely understand it. Especially in this draft.
He's gotten time at center but it hasn't looked great. Long term I think he's a winger who can fill in center if you need him