Why is it wrong, rather than entirely natural, to like something less when exposed to it more?

That's an oversimplification, I think, which obscures the differences between my example cases and yours. There's a difference between novelty being one's primary reason for liking something and that wearing off after a while, and on the other hand lack of novelty being an actively negative property which outweighs the thing's significant non-novelty-related good points. And there's a difference between getting sick of something because you or someone near you has been using it for a purpose outside its intended one (test tracks), and getting sick of something solely because it was being used in its intended way and was a victim of its own success. Finally, there's a false dichotomy implied between "wrong" and "natural": not everything natural is good, and it's perfectly possible – and reasonable – for something to be both natural and wrong, or (as in this case) both natural and something that irritates me and that I wish happened less.

I wouldn't be nearly so annoyed – perhaps just a little sad if I realised it – if after years of exposure to Bembo I had stopped enjoying its elegant italics, stopped noticing it at all, and now just treated it as basically unremarkable background to the actual text printed in it. What I dislike is that it's managed to start having a negative emotional effect on me, and that's by no means an obviously inevitable phenomenon.