A simple analytical fluid dynamic model is developed for evaporative electron cooling in a low-pressure decaying plasma and compared to a two-dimensional simulation and experimental data for the particular case of argon. Measured electron temperature and density developments are fully reproduced by the ab initio model and the simulation. Further, it is shown that in the late afterglow thermalization of electrons occurs by coupling to the ion fluid via Coulomb collisions at sufficiently high electron densities and not by coupling to the neutral background.