They shouldn't...
Absolutely not. But, any time you lose an elite player to free agency, regardless of position, you're not going to be as good.
Would St. Louis be as good if they lost O'Reilly? No. Or Perron? No.
Edmonton and Toronto lack defense for sure, but they have many other holes in their roster as well.
I'm not arguing that you shouldn't build a team around an elite defensive core, I'm arguing that you can build at team around an elite offensive core. But ideally you build a team with a bit of everything. Think about Tampa, with an elite center (Point), winger (Kucherov), defenseman (Hedman), and goalie (Vasilevskiy)...