That is a lazy excuse as the assets from both huds come from the exact same source, and you could literally reskin the old hud to look very similar to the new one. Obviously some changes would be in order, but this is by far the most desirable route.
You simply do not ever change a games HUD unless you have a damn good reason to do so (for example, lets say the old HUD was literally broken and didn't work properly). And even then you must ensure that the newer HUD more or less resembles the older HUD, especially in a game that has been around for years and has never changed its HUD in a long time. Drastic changes to the user interface in any form of online game generally causes uproar because people have grown used to the old system to the point that changing it would make the game almost alien to them.
I for one will not be playing league until they either give us an option to move parts of the HUD around and customize their size, or gives us the option to use the old HUD.