Hi there. You don't know me, but Alaska Airlines and Cathay Pacific certainly do. I've maintained MVP Gold (and, since 2011 when the new tier started, MVP Gold 75K - 75,000+ miles per year) on Alaska, and Diamond (120,000+ miles per year) on Cathay Pacific for the last 9 years. About 2 million miles in less than a decade. I fly a bit... In fact, I just got back from Hong Kong, on CX882 that arrived this afternoon in Los Angeles.
I am curious about your claim of only 1 airline offering your flight. What city do you arrive in, Internationally? Short of North Korea, Turkmenistan (Tashkent Air really isn't that bad), or a handful of other fascist dictatorships, I can't think of an International city that would be served by just 1 airline.
As far as US travel goes, I can't remember the last time I bought a business/first class seat - exclusive buyer of economy here. And I cannot remember the last time I was NOT bumped up to business class (that's what comes with massive standing with the airlines). That means that, up to 2 days prior to my domestic departure, there were still business-class seats unsold - and they give them to high-mileage travelers like me, for free. If you want extra legroom, I suggest you consider buying extra legroom - it seems to be plentiful, at least on Alaska Air and Delta (Delta recognizes my status with Alaska) - my two choices for domestic flights.
As far as Asian flights, you must not mean any going into, out from, or within China. For example, China Southern flies A380s for some of its domestic flights, it has so many fliers. China Eastern, Xiamen Air, Shanghai Air, all pack out all the time. It does pay to buy business class in China - but it's relatively affordable.