We develop a dynamic, infinite-horizon framework to assess the impact of conglomerate mergers on innovation and growth. We consider a setting in which competing platforms and complementors jointly offer value to consumers. At each point in time, platforms invest in R&D and face a trade-off between compatibility and the risk of imitation. A merger between a platform and a complementor exacerbates imitation concerns, inducing selective compatibility strategies that generate double-marginalization inefficiencies for independent rivals. This, in turn, reduces rivals’ incentives to innovate and slows growth.