For decarbonization purposes, variable renewable energies (VRE) are widely and quickly deployed in historically fossil-dominated power systems. Yet, some fossil technologies are more suitable than others for integration with VRE due to their higher ﬂexibility. I utilize an analytically tractable model to study the optimal transition to a VRE-dominated sys-tem when the endowment of ﬂexible and inﬂexible conventional generators is rigid. I ﬁnd that the existence of inﬂexible fossil generators hampers early deployment of VRE. How-ever, deployment speed increases after VRE begin to substitute generation from inﬂexible generators, which happens after VRE and inﬂexible capacities strictly exceed demand together. At this time, the decreasing use of inﬂexible fossil generation is usually ac-companied by an increasing utilization of ﬂexible generators. Nevertheless, constructing additional ﬂexible capacities is only proﬁtable under restrictive conditions. By contributing to a better understanding of the impact of ﬂexibility on eﬃcient VRE deployment, this work may facilitate an eﬃcient transition process.