The open-source ecosystem relies on aggregation as a core business model, with complementarities among aggregated products and services constituting a central operational feature. Market power in such ecosystems may not manifest itself through high market shares or elevated prices within a single product market. Rather, it may operate through complementary relationships and generate cross-market transmission effects, thereby challenging traditional relevant-market analysis centered on substitutability. Addressing this challenge requires an analytical framework capable of identifying how market power is transmitted across complementary markets and how such relationships may evolve from strategic distortion, to directed transmission, and ultimately to their institutionalization in governance rules that serve private interests. Specifically, operators of open-source ecosystems may create bidirectional complementarity among nodes through tying, control connection leverage through exclusive dealing, and shift competitive harm from the distribution stage to the development stage. These practices may subsequently be entrenched through ecosystem governance rules, giving rise to a closed and potentially monopolistic structure. Because competitive harm in open-source ecosystems may result both from the forward transmission of market power and from reverse ecosystem lock-in, antitrust analysis should move beyond a substitutability-based framework focused primarily on cross-price elasticity. It should instead consider a modified GUP PI-type metric incorporating diversion ratios, relative value, and non-price harm variables in order to assess the intensity of cross-market transmission and the strength of exclusionary incentives. Such an approach would redirect regulatory attention from formal openness to substantive openness and support the coordinated development of effective competition and sustainable innovation within open-source ecosystems.