Your logic blows my mind quite honestly.
(probably not right but i'm guessing how they go about this)
GT5 is really a gigantic code set. To fix somethings they need to change that code in places, so they may screw up in other places as well. And to add stuff, they need to add into that as well, which may screw up things as well.
So, right click and view page source here on GTP, now imangine that just with probably about 200x more text, that's probably what GT4's coding was like, which had less power to use, less features, no internet connectivity, and was probably simpler. Just Imangine how GT5's is.