In order to meet the exponential increase in wireless demand, new technologies are being considered for next-generation Wi-Fi systems (e.g., IEEE 802.11ax). Among these technologies is the adaptation of clear channel assessment (CCA) thresholds for high-efficiency (HE) stations (STAs) according to the beacon's received signal strength indicator (RSSI). The motivation behind this approach is to enhance the network throughput by improving the spatial reuse (i.e., allowing simultaneous transmissions from nearby STAs). There exists an inherent tradeoff between increasing the network throughput, via adapting the CCA thresholds for HE STAs, and maintaining fairness between legacy and HE STAs. In this paper, we provide a theoretical framework to evaluate the aforementioned tradeoff. We also propose a centralized fairness mechanism (CFM), in which STAs switch between an adaptive phase (CCA adaptation is allowed) and a fixed phase (legacy and HE STAs use the same CCA threshold). We formulate an optimization problem with the objective of determining the optimal switching strategy that maximizes the network throughput while maintaining a lower bound on per-STA throughput. Finally, we validate the proposed mechanism using simulations.