Can someone explain the constant focus on expanding the exposure of the NHL in the States and elsewhere. Is it's very existence at stake if it stays as a niche sport?
There are a few reasons people talk about this. For more media attention (particularly for U.S. fans). Also more fans will lead to more people who play hockey which will lead to better (on average) hockey players in the NHL, which leads to a better and (potentially) more enjoyable product.
Also, there are a few people who have interests here (such as myself) in marketing, business, law, or other related fields, and as such find the conversations interesting.
As a hockey fan in Canada, I could care less if it never expands again or even if it retracts a few teams, it will still be huge in Canada and popular in most of the current US cities who have teams.
I imagine we would still see the same level of play if the boys were only making $100,000 a year.
Thoughts?
Yes, but if the sports starting shrinking in popularity (and as a result, revenues) us fans will start seeing that reflected on the on ice product.
If a player is only earning $100k a year, there would be a huge decrease in the number of players trying to become hockey players. Some players are intelligent and talented enough in other ways that they will be able to earn as much or more money than that in another career -- one that wouldn't be so incredibly challenging to get your break in like the NHL is. Many junior players or NCAA players who aren't a "sure thing" will opt for getting a college degree (or finishing in the case of NCAA players) and getting a more traditional job.
Take players like Mark Messier, Patrick Roy, Brett Hull -- players who weren't seen as future stars or superstars when they were drafted -- and add the reality that they have a slim chance of maybe making $100,000 a year playing in the NHL for a (very) few years, and they quite possibly (and, imo, likely) decide that the NHL dream isn't worth the risk of trying for years just to get no where or, even worse, getting a career ending injury and having nothing to fall back on. They may decide that getting a college degree and going to work is simply a wiser decision (even if they dream of playing in the NHL).
Also, many players had to decide which sport to go with, since some of them were roughly equally talented at more than one sport. Deciding between baseball, football, soccer, basketball, etc and hockey will look a lot easier if "making it big" in the NHL nets them a whole 100k a year (and again, the increased risk of injury in hockey will factor into this decision too). Even if you raised that salary mark a bit above 100k, you're still going to see people going with other safer or more rewarding options.
Lastly, and most obviously, if players are only earning amounts in the hundreds of thousands, we are going to see a huge drop in european players in the NHL. Why play in the U.S. or Canada for less than you can make at home, although other league's salaries may drop since it wouldn't have the level of competition over players it does now with the NHL's massive contracts (ie, RSL). Not only that, North American players will suddenly be like the European players are now, going over to European leagues trying to get the big pay checks they can receive there. Even with the significant history the Stanley Cup has, money speaks to a lot more people's dreams.
The only benefit I could see is that the players who are in the NHL will likely be playing because they truly love the game, not just because they are good at it.
Now, obviously, you picked an extreme salary drop with $100,000, but the point will still hold true, to a lesser extent, with any significant drops in the salary NHL players make on average.
Seems to me that daver's own opinion is simply not caring if the NHL expands again. I don't see why you have to automatically append the "hockey belongs in Canada" idea to the end of that. daver stands alone though with the $100,000 a year comment. 
