actually, I think success in both places was important (and a difference; witness Gentle Giant who did better pretty much everywhere else but their naive country.
Also, for better or worse, at that time breaking into the USA was almost always the goal of bands from abroad, and I would not underestimate its value in the case of Yes...or any other group from the time, for that matter. In terms of potential sales, at a time when a group playing this kind of music could actually sell multiple millions of copies, access to the burgeoning American market was key.
It no longer is that way, of course - access to the US is much more difficult and expensive, so there's an increasing number of significant artists who, more than capable of success in other places, have decided that, as you say, success in the US is, indeed, no longer the be all/end all. But back in the '70s it absolutely was - representing the difference between solid commercial success and a potentially much, much bigger audience.
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