That was more or less the point of Tolkien's Lecture "The Monster and the Critics". He compared it to building a tower out rocks that came from a historical ruin. Everybody was interested in taking it apart to see where the rocks came from, but he felt it had value in considering it as a single creative work in its own right.
It's also the kind of idea that Tolkien was in a unique position to entertain. If one man can create an entire mythology spanning The LOTR, The Hobbit, and The Silmarillion, there is no reason to suppose that another man cannot achieve a similar feat with the Beowulf.
According to the article, doubts about single authorship began to be raised in the 19th century. This was a time when a lot of people thought that they were living at the pinnacle of history, and that cultures of the distant past must have been strictly inferior to the modern one. Troy could not have possibly existed; no single person in such a barbaric age could possibly have produced great poems like the Homeric epics -- or Beowulf.
Well, we found Troy. We also found evidence of great devastation at Troy right around the time when the Homeric war supposedly took place. It seems that people of the distant past did possess the ability to tell a great story after all, moving freely between history and mythology, filled with allegory and philosophical depth. Just like Tolkien did, but hundreds or even thousands of years earlier.
