Not to take anything away from Emerson. He was a fantastic amateur player. Probably the best amateur overall from 1963 to 1967. And as mentioned, was equally if not more adept in doubles. And yes, he could only beat the guys that could face him in the majors. But he was not quite as good as the best pros of the day, who were not allowed to play in the majors, who also played with similar equipment. For example, when Rod Laver first turned pro, after winning the 1962 Grand Slam as an amateur beating Emerson in 3 of the 4 majors, he was beaten like a drum by pros like Rosewall and Hoad for most of 1963. I think Hoad beat him 8 straight and Rosewall 11 of 13. Laver admitted he was simply not in the same class level. But eventually, his level rose and by 1965, he turned the tables and was clearly the #1 pro.
Emerson could have also turned pro, as Laver did in late 1962, but chose not to. He was making enough under the table to keep himself going. So without the best pro competition, he was able to win all those majors, beating fine players like Roche and Stolle, which kept him in the record books and kept people like Sampras chasing his slam count for years. So even though Emerson was a fine singles player, and probably one of, if not the best doubles players, this is why I can't place Emerson among the greatest singles players, even though he has the slam "numbers" to be near the top.
Of course, everyone is entitled to their own opinion...