Not that I'm advocating 7x, but to be fair to those who do, their argument isn't that the fish can't see it, it's that it allows a more drag-free presentation, and that for nymphs, that it sinks better.
The former can just as easily be accomplished by a longer leader and/or a slack-line cast. A heavier fly and/or a tuck cast takes care of latter. Or you can use a finer leader and lose more flies and fish.