Even people who were heavily watching anime in 2003-2004 are unlikely to have seen this, on average.

Stellvia, Scrapped Princess, Last Exile, Ikkitousen, the first FMA, Chrno Crusade, FMP:Fumoffu, Planetes, ROD TV, Onegai Twins, and Kimi ga Nozomu Eien were all airing at the same time as Kaleido Star. From that far back, all but the most prominent series are completely forgotten about.

The series more likely to be mentioned as a quality overlooked series from back then are probably Gungrave and Kino no Tabi before Kaleido Star comes up.

Kaleido Star was easily overlooked then. I'm honestly not aware of anyone who actually watched it while it was airing (myself included), they all picked it up after it was completed.