I think it is possible to have a large output and retain a high level of quality. One need only look to classical music where nearly all well known composers wrote enormous amounts of music relative to modern day artists, and yet the quality levels rarely dropped significantly. I think this reveals two things, firstly that metal lacks a formalized technique and so the artist must spend a significant amount of effort trying to "reinvent the wheel" every-time they compose something. Secondly modern day artists are not able to retain a high level of artistic insight and inspiration in the surroundings of the modern world. Rather than growing in wisdom and insight as they age, like many classical greats (Bach, Beethoven, Brahms and the list goes on), heavy metal musicians tend to lose their passion and become absorbed by the triviality of the modern world so that they are pale shadows of themselves as artists by the time they reach their 30s.
These composers, though prolific, were primarily known as composers rather than performers. When they did perform, it was generally smaller solo pieces. The occupation was also much more limited - by access to instruments and education, by the selective and sort of elevated status of being a composer, and access to audiences being much more controlled than it is today. Those who were brought into the profession of being a composer had both their own benefits, and a massive comparative advantage to others. Few instruments of outstanding and lasting quality were made, and they were in the hands of a more selected class, while folk instruments were of lesser quality - both more difficult to play and master, and less precisely intonated so they would sound much less consonant and harmonious together. Composers had the luxury of an orchestra of trained musicians, should they prove to be skilled to the point of being able to compose for such an ensemble. Finally, the venues provided an enhancement to the music in their acoustics and experience, something that gave their music a much more grandoise theater than those without access to it.
The much higher bar to entering formal/written music composition was both selective and enhancing to the greats of those times. It is important to remember though, that we are looking at hundreds of years and relatively few composers - metal has only been around for as long as JS Bach alone was composing.
With the advent of recording technology, the original performance, rather than the manuscript, has become the defining format of music. The limitations to making music have become much less selective, and music is no longer "benevolently" sponsored, but designed to be merchandised as a product and a brand name. Music of the last ~100 years has been made under different circumstances, skewing the comparison of prolific modern artists to famed composers of the past.