You can't judge the quality of composers by how "easy" it is for Mozart to write brilliant music quickly. That doesn't mean anything by itself. It's a gimmick. Mozart is a pantheonic composer and that he got there in so short a life is commendable. But prodigious is what it is, no more, no less.
Also fun fact: Plenty of the greatest artists give their predecessors the full benefit of their legacy. It's just common courtesy, standing on shoulders of giants, and all that.