1) Actually B737 ,it is cost saving.They had to change the wing,but it has nothing to do with the rest few hundreds or more components..
The wing is not the only change,but there are many items that remained the same.And it's cost saving even if you keep the buttons while changing the shirt.
Why do u think the source off light is useless? It informs u that you may be on only one source for both busses,or that something has changed from what you think it's under way on electrics operation.
And it has to be amber in color...