What's all this whining I hear about Sombra not being bad enough? Or getting enough screen time? Or not being developed enough?
Calm the fuck down.
He's going to be back. The lack of exposition confirms this.
When Cadance's magic shield sliced off his horn, we saw the severed horn piece act on its own (melting into the ground). Later we saw Sombra repair his horn completely. What has been established? That Sombra can regenerate (his horn anyways).
What happened the last moment we saw Sombra? He got obliterated into shards by the crystal heart magicyness... and we saw his horn (intact) fly away (towards the viewer).
He'll be back.
Besides, the episode wasn't about Sombra. It was about Twilight Sparkle.
What happened at the end? Twilight was allowed to advance in her studies. Celestia and Luna are preparing her for bigger, badder, showdown that has yet to happen.
My guess is that we're going to see a winged Twilight at the end of this season.