The reason we can still observe light that's billions of years old, is the universe has expanded since the events generating the light occurred, allowing us a chance to see what happened.
Only a small part of the most scientifically-significant things we need to see happened in what is now the visible band of light.
In an engineering sense, the Webb scope is fascinating because it will be placed in an orbit that is out past the moon (a "lagrange point") where it can remain in shadow most of the time. It also has a layered structure which shields the instruments from sunlight, allowing for a clear view of the infrared band ("black body radiation" aka "heat"). Any heating of the instruments would obscure the light from the observations too much to get good data - hence the need for effective shielding from sunlight.
The scope only goes into sunlight long enough to charge solar cells on board, then rotates back into shade again. It is an engineering marvel.
* Even though light is not blocked by space (photons do not interact with empty space much at all) over time, photons do tend to streeeeeetch out, which means the frequency of the photons goes from the shorter gama, visible and ultraviolet spectrum into mostly infrared. That's why a scope needs to "see" in the infrared and remain so cold to be of maximum value to science.
