I agree to a point. I think Metallica had several things going for them - good songs, well written lyrics (that were never one-dimensional like always singing about satan etc), and most importantly, they were better live than anybody else.
I loved Megadeth, and I think Rust in Peace is one of the best albums of all time, but they couldn't play live to save their lives. They had some of the worst 'sound presence' of all those thrash bands of that day.
I agree though that there were better other thrash bands (if Metallica were ever thrash). Sepultura with Max Cavalera were untouchable. Even today, albums like Beneath the Remains are just astounding.