Originally designed for five years of plasma operations, the JET Far Infrared (FIR) interferometer/polarimeter diagnostic system is still operating at its full capabilities nearly forty years later and in ITER relevant conditions, albeit with significantly lower neutron fluences and for several D-T campaigns. The original design changed substantially over the years, just to give two examples: the introduction of the divertor significantly reduced access and the number of laser beams; D-T readiness required double vacuum windows that greatly reducing the laser signal level through plasma to 5% but the diagnostic still worked, due to the excellent dynamic range of the detectors. This paper will discuss invaluable lessons learned designing, operating, optimising, and enhancing such a complex system and how these can these used for developing the new class of laser-based diagnostics for the next generation reactor grade machines.