Yes, the case is used as a heatsink. But the “tested ambient temperature” indicates that up to 70C ambient is allowed. However as the temperature increases, the MTBF might decrease. The rated 200000 hours is at 25C. If the ambient is under 30C most of the time then I would expect it to run for a very long time. I have not seen an issue of Mikrotik choosing cheap capacitor manufacturers. The CRS305 is cheap enough that you can keep a spare sitting next to it programmed but turned off.
The only way to have something better, would be for your ISP to provide the two connections to you - even if this is just from a switch that they maintain and would replace within hours of failure.