I don't get why people think Arya's show time was wasted. She wasn't just training to be faceless, she was training to kill from the shadows, to blend in, to completely lose herself in a part and act like someone else; she can still do that without changing faces. Because she spent time learning these things, she'll now be a much more believable badass fighter-assassin, especially since she can now use the force to guide her lightsaber. She also had some character development - she started with this gigantic list of names of everyone she wanted to kill, that was her only motivation, now that's gone. She now knows herself much better, who she is, and realizes that not everyone on someone's list deserves to die (she started learning this with The Hound of course, but has now had it reinforced).
Sure it could have been handled better, but I don't think the way they ended it means the whole training sequence was a waste. She is now a character who can meaningfully affect the world.
