Usually agree with that. Altho, there are exceptions. Anybody who enjoys the Iliad in a modern translation like Fagles, should also try Alexander Pope's 18th century translation. It remains unequaled in its ability to render the Iliad into majestic, poetic English that is worthy of the subject matter and Homer's poetic original. For example, a comment about Pope's translation from a NYT review: "The thing that best distinguishes this from all other translations of Homer is that it alone equals the original in its ceaseless pour of verbal music."

Similarly with the Bible. I normally read a modern translation, ESV. But there is poetic simplicity and power in the 1611 King James translation that is unmatched by any translation in our time. It is still worth reading, and probably my favorite translation of all time, despite a fair number of underlying manuscript and translation mistakes in the original KJV. Robert Alter, a leading Old Testament scholar and translator, has commented frequently in his books on the literary power of the KJV and its lasting influence on American and British writers.

In cases like these where you have a classic, masterful translation on an important work, and good modern ones, it's really worth the effort to read both. But it depends how much you care about the work in question. For me at least, there are only a handful of works like this where I'd bother to read the older translation(s) as well. So I'm agreeing with your general point, just saying there are worthy exceptions .:-)