That's not... the case, at all. It's baffling to see this bandied about like it's common knowledge. No, there is room for a lot of growth in Canada, or teams like the Canucks and Flames would still be bleeding like they were in the '90s. Hockey interest has grown quite a lot in Vancouver since 2000, mostly I think from immigrant communities (who've really embraced the sport). Just ancedotally in my circle of friends, roughly about half (bout 66% girls/33% guys) could give less of a **** about hockey. That doesn't mean they don't know it's there, but they're not paying customers of the NHL. In cities like Winnipeg, Quebec and Hamilton hockey interest is high-ish but many people don't attend NHL games and afaik many sort of 'follow' the game but don't invest in it as much as they might if a team were there. There's definitely room for hockey to grow here.
However you're not totally wrong. I think it's best said that the room for growth- the marginal $ they'd get out of one more Canadian hockey vs. a hypothetical American fan, is much higher in the latter case. There's much more room to grow hockey in the U.S. than there is up here. However, it's debatable whether franchises actually do that and whether or not it might be smarter (or dumber) to have more Canadian teams were growth probably moves faster due to its sort of cultural hegemony over us.
I think if Edmonton absolutely refuses to build a new arena then sure, they might move- it'd be tragic and stupid imo. but I could see it happening, but they
will get the arena done it's just a process and the NHL's contributing. You need the implicit threat of relocation. I don't think they'd want to move the franchise to Houston if they could move another one there or get an expansion fee out of a new franchise there, but you know it's not off the table.
Is Canada's national psyche so fragile that this is really a big deal?
Hockey isn't going to grow in Canada. That's the only point here. There is no one in Canada that has never heard of hockey or might be convinced to give it a shot via exposure.