Thanks for clarifying!
You make a good argument that Ebert's argument isn't sufficiently scientific. Ebert also doesn't provide any data for his hypothesis that the degraded qualities of movies is going to effect box office returns.
However, I think these are the wrong criteria for judging the article. I don't think Ebert's it's meant to be scientfic, nor is his article primarily about economics. I saw it as more of a love letter, which is very personal, very subjective. His last paragraph:
"I despair. This is a case of Hollywood selling its birthright for a message of pottage. If as much attention were paid to exhibition as to marketing, that would be an investment in the future. People would fall back in love with the movies. Short-sighted, technically illiterate penny-pinchers are wounding a great art form."
The primary purpose of the article isn't to explain the potential economic impact of the degradation in projection quality, or to provide scientific proof that the majority of people are affected. It's to say that movies look shittier, and to explain why - and to express how sad it makes him that movies are looking so shitty. I saw the remark about how people would be less likely to return to theaters as more of an expression of his unhappiness than as the central point of his article.
Personally, I found the article very helpful. I saw Thor opening weekend, and I was confused about why it looked so dim. It was disappointing to me to view something so graphically rich in a way that made it look it was 50 years old and faded. I didn't know what was wrong, but now I do, and now I know how I can attempt to see movies in a way that I'll like more.