Rival Wings is just a poor mode for the NA playerbase, really. It demands too much of people who already want to get by with as little effort or responsibility as possible. Gordon absolutely nailed it; the people "playing" modes like Shatter that can't even understand the basic premise of watching the map (or listening to callouts) to go after objectives are going to be absolutely dismal in a mode with several different objectives of varying priorities and a party-wide resource. If you couldn't be arsed to constantly pay attention to the map in any Frontlines mode, you are dead in the water for Rival Wings.

Nearly every complaint (unfounded or otherwise) I often see about Frontlines in general or CC is magnified in RW. "I don't wanna do objectives, I want to (try to) kill people:" then RW would blow your mind with the sheer amount of things going on. "My team sucks, we were doomed from the 1st engagement:" Enjoy having to wait-out like 10 minutes for the other team to break both of your towers and then the core in obviously lop-sided games. Or perpetually being down 1-2 mechs in your alliance because at least one party is not piloting mechs/gathering CE or is grabbing an Oppressor to shoot rockets (meant for towers) at players the entire game. "Battle High is broken:" Enjoy dealing with Soaring; it never goes away even if you kill them... Just to name a few.

Astralagos was removed because it was deemed "too large" by the devs; you spent way too much time actually mounting up and running to locations than actually participating. This is reflected in Hidden Gorge being more compact compared to it. Also it's middle area was incredibly flawed in that whoever controlled it steadily increased their entire teams' CE amount which snowballed into that team cranking out those mechs way faster than the other team. And I didn't forget how incredibly broken it was to plant a few healers on it and all they had to do was indefinitely stay alive and tag anyone trying to interact with the switch to cancel their attempts. If you didn't control the middle area or were actively in the process of storming it, you were almost guaranteed to lose. Hidden Gorge now has a periodic resource dump in mid, and a Goblin mercenary instead. The devs got rid of Astralagos to seemingly deal with these imbalances first, but like the way of squadrons and Egi glamours, they probably just want to forget about it instead.