Well the bottom pairing would likely be a bit of an adventure no doubt with players like Bouchard/Gravel/Bear having to be on there all season long.
Then again... like you say... with a great/elite 1st pairing and a good/decent 2nd pairing... there has to be a weakness somewhere and if it's going to be the bottom pairing... that's obviously the best place for the weakest links on D to be.
One of Bear/Bouchard(if he sticks the lineup) all season long on the bottom pairing would likely make for some minuses on that pairing at evens... but so be it. Gravel might be a little better than we think as well.
Of course if several injuries hit as well (which they likely will as they always do)... the Oilers lack some experience on D but it's hard to have extra depth at all positions and the Oilers D depth is likely better than their wing depth even after injuries are taken into account.
Lowe/Stanton would also be available for injury callups who'd both be fine for #7 pressbox sitters and as fill-ins as #6 dmen for short periods.
As an aside... what's the deal with Joel Persson... a 24 year old dman from the SHL signed to a 1 year deal by the Oilers but looks like he'll be playing in the SHL next season? Could he potentially come over at some point this season as well?
Yeah, I totally get the appeal of adding Karlsson (it's really hard not to like it, there's worse problems than having McDavid and Karlsson running around for 25+ minutes), but I just get leery about super top-heavy d-corps.
Personally, when I look at teams that have stacked top pairings and then a bunch of "ehhh he's alright enough", I don't see many that are legitimate threats. Pittsburgh is the most notable exception, but any team with Crosby and Malkin is going to cover up for defensive lapses. LA and Chicago are both slowly tumbling from relevance in part because their defensive depth is now described as "one star, and a bunch of mediocrity".
I know there's some recency bias, but I'd personally rather construct a team around a bunch of above-average to pretty good defensemen like Vegas, rather than one super-elite pairing and a bunch of guys who might break even if they're lucky.
RE: Your aside, I
believe that the Oilers now own Persson's rights after this season as an RFA, so while he's contracted in the SHL we can negotiate North American time next year, but I struggle with contract statuses!