Kinda late on here, but thermal paste usually degrades with temperature extremes. It depends on what paste you use but the factory paste is typically on the lower end of heat transfer performance but it can take a fair bit of temperature for 4-10 years depending on what they used. After that the degradation will probably leave you with 5-15 Celsius higher temperatures. Though it is important to note that it (probably) won't get worse than that- pastes are built to not turn into powder for many many years.
Higher performance after market pastes usually have lower maximum temperatures. The pastes degrade more quickly the closer you are to the maximum temperature. So a data center with core temps of 40c will probably not have to replace the paste for the life of the server (2-6 years). On the other hand, if you put a high end paste on an air-cooled consumer computer, you will get worse thermal performance after 1-3 years.
Factory pastes are usually applied with the screen method- so it usually doesn't leave any air bubbles like it might if you apply it at home (this also contributes to longevity). However, if the application was poor to begin with- all bets are off. The dish receivers say they are made in India, where labor is pretty cheap. It is quite someone squirts a little bit of paste before everything is sandwiched together.
