It does kind of turn out like that and is a bit of a bigger issue later, but overall Sora later grows enough on his own for his Heart to be worthy, which is how he materializes in I after being turned into a Heartless and being able to use a Keyblade without Ven's heart

Roxas is basically the biggest woobie in the series and 358/2 Days really explored that. Still, he's treated as a separate entity from Ventus. Sora 's heart actually accommodated Ventus' when he became "broken" at the end of Birth By Sleep and his body is somewhere else, waiting for his heart to return so that he may awaken again. Roxas was technically Sora's nobody in that respect, but with Ven's heart inside, which paradoxically disqualifies him from being a Nobody (although it later turns out Nobodies did actually grow hearts overtime after all).

So yeah, it may sound like a bummer, but a lot of it is very nicely explored in the series. The whole aim of Sora's journey at this point, after 3D, is helping bring back all those people who were not allowed to properly exist in one way or another