Pokémon Contests, since RSE, seemed enjoyable to me ever since. They have a place for me and it is something I find interest in when playing the main metagame. I agree that something extra could have been done to Pokémon Contests in the game to relate a bit more to the way it was done in the anime. However, I wouldn't rule out the possibility of it occuring once more in future games to come. Not to mention, it hasn't been confirmed that Pokémon Contests are completely out of the GSDS remakes, so for those who do enjoy it, then I wouldn't drop all hopes yet, but I wouldn't keep your hopes up entirely high either. There is still a chance at it showing up in Kanto since nothing much has been revieled about the second region as yet besides a few screenshots and videos about Kanto.
As for the Pokéthelon, I can't say I am guaranteed to liking it because all I have seen are a few images and trailers on the subject. However, I cannot deny it looks intriguing and interesting no doubt, but I will have to actually experience the Pokéthelon before I can place my judgement on it like I have with Contests. So, for this matter it depends on overtime if I will prefer this new feature over Pokémon Contests. Although, it does look pretty cool at a perspective such as this.