I dunno.
The negative is that you'd have a ~$14M AAV guy with 100pt potential just collecting dust on a team that should be actively trying to be as bad as they can post-Sid/Geno in order to try and land top-3 picks and build through the draft. You could try and move him once Sid retires, but you'd better bet his production and value's gonna plummet on this team, and it's gonna take heavy salary retention to move even with a ballooning cap ceiling.
It's a move that screams "ooh shiny" to me.
He's about to be 28, and this team seems hellbent on clinging to those 10-14th overall picks until Sid retires in 2027. So you're talking about Marner being 30 by the time this team genuinely starts to suck shit and collect top prospects, and assuming they don't miraculously fall ass backwards into another generational talent, probably a solid five years of building out a solid roster. There's no real positive to chasing Marner at this stage of the franchise, imo. /shrug