I think we need both a better colonial management (which may come from a stronger and richer Netherlands by retaining Belgium, but who knows) and no disruption of colonial rule like Japanese occupation did during WW2. The later radicalized political atmosphere in Indonesia. Previously, most native politicians pursued independence through cooperative route. But Japanese occupation brought to Indonesia anti-western nationalist agitation, devastation to local political and economic structure mostly due to post-invasion management, militarization of the youth, Java-favoritism, and the chance for PKI to rebound.
I've been curious about how would the retaining of Belgium would've influenced Dutch colonial policy. Obviously, they would've had more money to fund Etische Politiek, but then again, there still was the lack of enthusiasm among the local administration. Then again, arguably the lost of Belgium may have played crucial role in shaping the Netherlands' economic policy throughout the 19th century. Don't think it will prevent Kultuurstelsel, but maybe less dependence on it and thus, less harsh version of it ? I'm not sure though, but still, more money to spend to build the colony will do something...