https://c3.ndc.nasa.gov/dashlink/static/media/other/Observed...
The long story short is that there are byzantine failure methods which prevent a 50 year computer. A sample:
- Capacitors can act as bullets
- Forced air cooling creating water
- The smaller the parts, the greater the chance they'll transmute to another part. Even, or especially in solid state parts.
- Digital isn't (i.e. 1 isn't really full voltage, and 0 isn't really no voltage).
- Thermal expansion matters, even for ICs on a board.
- Wire length, and the position of sensors on that wire, matters.
A 50 year computer would probably have to be one in which each part can and is replaced on a schedule. And the faster the computer is, the more often parts would need to be replaced. Additionally, if we want 100% uptime there would also have to be sufficient redundancies to ensure that the computer could continue operating during failures or replacements of components.