I can understand that view from the outside, but nothing could be further from the truth. The city is hockey crazy for the Sens - they are close to the top revenue per capita in the NHL. The city punches way above it's weight class in terms of fans and revenue. Certainly per capita, they actually generate substantially more revenue than either Toronto or Montreal. And that is despite having a decent subset of other teams fans here too.
to that point, there is certainly a solid base of original six fans here. The city loves hockey, and many fans were fans of team before the Sens arrived. I know for my generation, we all grew up with a favorite team given we didn't have one. Most people changed, and some didn't (not just Montreal and Toronto - see a regular season game for the Wings, Bruins, Hawks or Rangers - tons of local fans). Some have Sens as their second favorite, others hate them (mostly Leaf and Habs fans, for natural fun reasons). I get how some people see this as a bad thing, but really it is just fans who were deep rooted in their original team, and couldn't make the switch. And again, the city is hockey crazy. The different fan mix should be celebrated to a degree. I no other city is water cooler talk on hockey so intense and heated.
As to your assertion that "Most fans" cheer for someone else, that's obviously ridiculous. I'm sure you were just exaggerating to try and make a point. More than 90% of hockey fans are going for the Sens I'd guess, but that still leaves probably 50,000 people who are fans of other teams here.
As to the non-sell out, as a season ticket holder, I can tell you this lies firmly on the team. They have consistently tried to use playoff tickets to get fans to sign up for season tickets, and for those on a budget, it just doesn't fly any more. And if they don't sign up, making them pay 2.5 times what others are paying, just rubs people the wrong way (now that they've figured it out). The Sens could have sold out the building 2 times over if they hadn't tried charging $400 for a $200 seat. That's a lot of $'s to expect most fans to pay, especially when they know others paid less than $200.