The accumulation of post-consumer plastic waste poses a critical challenge to sustainable urban infrastructure. This study evaluates the nonlinear thermomechanical behavior and lifespan of a street light support made from recycled high-density polyethylene reinforced with 30% glass fiber (r-HDPE/GF30%). Using a multiphysics computational framework that integrates Finite Element Analysis (ANSYS) with large deflections and temperature-dependent plasticity, three geometric configurations are compared: circular (G1), rectangular (G2), and variable elliptical (G3) cross-sections. Under severe loading conditions at 100 °C, the results demonstrate that the conventional geometries (G1 and G2) exhibit structural instabilities and a state of severe instability, with Safety Factors (SF) of 0.26 and 0.29, respectively. In contrast, the optimized G3 configuration mitigates thermal softening through efficient redistribution of internal stresses, achieving a factor of safety (FS) of 1.57 and a controlled maximum deformation of 149 mm. A sigmoidal decay kinetic model, calibrated with the numerical results, projects a service life of 10.4 years for the G3 profile compared to 5.8 years for the rectangular design, representing a 79% service extension. These findings validate that geometric optimization is a strategic requirement for the viable implementation of recycled composite materials in long-life portable infrastructure, promoting the transition to urban circular economy models.