For certain destinations (a famous museum?), there might be simple solutions like just restricting the number of visitors and managing their flow more efficiently. For cities, there are no easy answers. This is just another outcome of the huge increase in human population. You might be able to expand highways (up to a point) to accommodate more cars, but you can't expand downtown Paris to accommodate more people, without damaging its character. I think we should not be too judgmental of those who suffer crowds to visit popular sites - in the end it's just a question of personal preferences in the search for a meaningful experience. Some can find it despite, or perhaps because of the crowds, while others will find it necessary to stay away from these. I suppose that most here on this forum belong to the second group.
What's more important is to remember, as a tourist, to respect local ways, be friendly and humble, try to learn some basic words of the language, and keep your voice down to reasonable levels.