Carlson may not be irreplaceable but what's the list of players that could single-handedly replace him? Burns, Subban, Doughty, Karlsson, Klingberg, Pietrangelo, Byfuglien, Jones, Weber & Hamilton. Maybe Dumba, Ekblad, Parayko or McAvoy could grow into it as well. So that's 14 defensemen, with some questions as to whether some would be capable of it just yet or, in the case of Weber, whether the future return would necessarily be comparable. They could attempt to split up the responsibilities between more of a PP specialist and then another PKer type but that weakens the overall quality of the blueline and second pair.
I think he's a must sign. We can make ideal criticisms of his game--he's never been an overly strong possession defenseman 5-on-5 compared to the very elite--but it's not like Niskanen or a fairly generic top 4 RHD are capable of being the guy on PP1 just because of the talent around them. There's no Reirden type internal replacement to where they can afford to coolly back away should negotiations force them into a disadvantageous position. It has to get done. I'd hope that the OEL contract would be enough, if not 8x8. Anything more than that and it's really pushing it. That would already be a top 15 AAV. I think he conceivably could make a case for Subban type money but not here.
This was my point in the Roster Building thread. I didn't have Weber, Klingberg, or any of your "maybes" as potential replacements but regardless, all of those guys will cost assets + cap space instead of just the cap space for JC74.
Big, strong, mobile RHD who can QB a PP and eat PK minutes in addition to playing 20+ ESTOI do not grow on trees and they almost never hit the free market. Pay him.