The reason a new result in science is exciting is because it proves the existence of a postulated possibility. Hypotheses are one thing, empirical evidence is quite another. It's not a matter of arrogance not to assume something exists until proof is found; in fact, to act otherwise is irrational.

We know the sun's fuel is hydrogen and helium because we can analyze the spectrum of light it emits. Each element emits a very specific set of discrete spectral lines, and from the sun's spectrum (or that of any other star, for that matter), we can determine its composition. Interesting fact: helium was discovered in the sun's spectrum before it was ever observed on Earth.

So, it's extremely improbable that the sun's fuel would be made up of unknown elements. The basic elements follow an extremely well-known progression, and at this point we have the complete periodic table of "natural" elements, along with a few dozen "synthetic" elements that have been generated in laboratories. Hydrogen and helium are the lightest elements (their nuclei contain only one proton, and two protons + two neutrons, respectively), so they're the most abundant in the universe.