I have good faith in the writers that they don't construct their stories in a way that panders to every whim of the fanbase. I know they do love to sprinkle in a lot of fanservice, but come on now, are we so spoiled that we believe even major story arcs are resolved just to appease us?
Think about the theme of the season. There's been tremendous focus on Cutie Marks. On top of that, the CMC have been growing up steadily over the prior four seasons and they're just not the same mischievous fillies they were in the first season. This show has defied expectations of a slice of life series and put the kibosh on the status quo before, what with Twilight's ascension to alicorn princess and all. Realize that the writers come up with their ideas as they go, because they have no control over how many seasons the show will continue for. They plan out the main ideas for each season before the episodes even get written. So chances are a lot more likely that they just decided this was as good of a time as any to have the CMC earn their marks, and I think it was a good call. I was apprehensive at first, but after seeing how beautifully the Crusaders' arc was concluded, along with the brilliant potential for new kinds of adventures for the fillies, I'm glad the writers decided this was the season. This really isn't any different than Rainbow's inching forward to become a full-fledged Wonderbolt or Rarity's progress in the fashion world. MLP is developing its characters and building on its world to keep things fresh and exciting. This isn't the end of the Crusaders as we know them. It's just a new start.