The challenge of enhancing long-term efficiency and reliability in power systems is tackled through Transmission Expansion Planning (TEP). This study evaluates the actual failure probabilities of system components within the TEP framework, with particular attention given to the impact of plug-in electric vehicles (PEVs) and their integration with demand response (DR) strategies. The analysis explores how the growing integration of PEVs into the grid affects system loading and, consequently, the loading and failure probabilities of transmission lines. Due to their capability to charge locally, PEVs impose less strain on the transmission infrastructure when compared to Grid-level energy storage systems (ESS), such as pumped hydro storage. To quantify the benefits of DR strategies, their implementation cost is incorporated into the TEP objective function and compared against the DR contributions of other consumer categories. Additionally, effective demand management may reduce the need for constructing new transmission lines by decreasing the failure risk of existing ones. Thus, a comprehensive valuation of the transmission network is essential and must be reflected within the objective function. The proposed framework is validated using three case scenarios based on the IEEE 24bus RTS, demonstrating the practicality and efficiency of the suggested approach.