Big ice hockey is extremely different from small ice, and you can finesse your way out of most situations. There is no avoiding the battle space in the NHL. It's why little DaCosta left the Sens as a bit part and wound up a top scorer overseas. Karlsson is a super elite offensive defenseman, and partly because he can't defend around his own net on small ice, he is a career minus player.
And you're right - 26 years ago, the Rangers with 2 very offensive defensemen won the Cup, and one was so elite he led the playoffs in scoring. Could happen again, but again, on a budget team, and 2 undersized guys, I don't see it. Soft rosters never win the playoffs.
Daigle was a high speed, high skill smaller player who couldn't handle the middle of the ice, and basically flopped. He wasn't even forced to battle in the slot except for scoring opportunities, and he went from junior star to very average because he was only effective on the edge. Now imagine a defenseman with the same profile forced to battle in his own zone for 10-12 minutes a game, giving up the size factor on every battle. Logically it could be the same outcome. If the scoring is not elite, there is little to fall back on. A few soft, skilled forwards on a roster can win a Cup. A few soft, skilled defensemen and it doesn't happen.