I'd argue he's been the best defenseman in the league over the last 4 seasons, and injuries have been why he hasn't won the Norris. If he's healthy in 2020-21 and doesn't miss 11 games, he wins over Fox, no question. If he doesn't miss 22 games last year and finishes with 90+ points, there's a chance he wins it despite Karlsson scoring 101. He also put up the best PPG rate this season, but missed 5 games so Hughes finished with 2 more points. Without the injuries, that's plausibly 4 straight Norris wins, to go with the Conn Smythe and Cup.
He's also 0.12 PPG ahead of the 2nd best D over the last 4 years combined. Over the same period, McDavid is 0.19 PPG ahead of the 2nd best F. I'd say that's pretty similar domination over multiple years.