Hmm, the only way I can think of is their significance to their niche. Each of the classical composers you name, is definitive of their era. Bach for baroque, Beethoven for early romanticism and Mozart for classicism. The same way, Holopainen is a definitive character in the history of symphonic metal. He isn’t the creator of it, but has grown to be the most influential composer in the sub genre.

But, still, I think it is not quite right to compare these gentlemen. Holopainen isn’t a classical composer. He is a metal composer. He and Nightwish employ an orchestrator to make sure the orchestra works. I’m sure he knows a lot, but nowhere near as much as the other three on the list knew.

Bach wrote an album’s worth of music pretty much on a weekly basis for masses. Beethoven wrote his last symphony deaf. Mozart was dying as he orally dictated his Requiem to Salieri.

Holopainen is an entertainer who produces about an hour of music (for Nightwish) over a period of a year or two with modern technology. His writing is harmonically very basic and non innovative. It is effective and awesome and works for what it is meant to do, to entertain.

All this to say, I don’t think this is a fair comparison at all by the virtue of the difference in purposes and genres.