EU-DEMO is a fusion power plant concept being developed within EUROfusion. Building on experience gained on recent fusion experiments and the development of ITER, its objective is to demonstrate the feasibility of reliable, power-plant-relevant operation of a fusion device. In contrast to ITER, a power plant is expected to host only those diagnostic systems that directly contribute to plant monitoring and control. Several of these systems are based on optical principles, and the condition of the first mirror heavily influences the performance of the systems. Previous studies have investigated how the mirror surface is affected by erosion driven by high-energy neutral particles and the deposition of various impurities. In the present study, it assessed how the expected lifetime and performance of a conceptually designed, gas-flow-protected optical diagnostic system changes as selected design parameters are varied. To this end, the extent of mirror-surface erosion as a function of gas pressure and mirror position was assessed. The results indicate that, for the investigated cases, the imaging quality remains essentially unchanged, and the predicted erosion magnitude suggests only minor first-mirror surface degradation. In addition, a deliberately conservative worst-case scenario was evaluated by introducing a 20% diffuse reflectivity component to represent strong surface roughening; even under this assumption, the system maintains its imaging performance, while the collected power is reduced as expected.