This paper integrates research on child simultaneous bilingual acquisition more directly into the heritage language acquisition literature. The child simultaneous bilingual literature mostly focuses on development in childhood, whereas heritage speakers are often tested at an endstate in adulthood. However, insights from child simultaneous bilingual acquisition must be considered in heritage language acquisition theorizing precisely because many heritage speakers demonstrate the adult outcomes of child simultaneous bilingual acquisition. Data from child simultaneous bilingual acquisition raises serious questions for the construct of incomplete acquisition, a term broadly used in heritage language acquisition studies to describe almost any difference heritage speakers display from baseline controls (usually monolinguals). We offer an epistemological discussion related to incomplete acquisition, highlighting the descriptive and theoretical inaccuracy of the term. We focus our discussion on two of several possible causal factors that contribute to variable competence outcomes in adult heritage speakers: input and formal instruction in the heritage language. We conclude by offering alternative terminology for heritage speaker outcomes.