Just to clarify, nothing was trashed. Several of the suggestions for the mode were implemented, such as the specially designed City map (which was created with Helicopters in mind) and in general more maps with terrain cover, new objective types, lobby improvements and several other changes. Which are often looked over and ignored that they even came from player feedback, because its easier to focus on what was not implemented and not consider the reasons why it could not be implemented. A big one of which was the Helicopter Arcade Assault itself was a suggestion that came directly from player feedback and is played by a decent number of players.
What I believe you are referring too, in terms of what was not implemented, for reasons which were explained in several past Q and As and answers on the forums here were the suggestions like a BR based matchmaking for Helicopter EC so certain BR helicopters could not face lower rank helicopters. This feedback was not "trashed" but thoroughly considered in several iterations and possible outcomes. Simply put, it was explored very deeply and the devs were very much aware of the reasons behind this, however without the necessary population to actually sustain such a change, it was not actually possible to do so without resulting in effectively preventing any form of match being filled. The framework required to sustain such a system did not exist. Helicopters are inherently used more in Combined Battles and most player usage was there. Splitting the existing Helicopter EC population down further in any capacity would have had even more negative impact on the mode.
Helicopters are somewhat unique in game being the only vehicle type that is not really designed or intended from the outside to fight each other. In that a general sense, aircraft are designed to fight other aircraft, tanks are designed to fight other tanks and ships are designed to fight other ships. Helicopters themselves, both in reality and how they function in game are inherently a supporting class. Imagine if every aircraft in game was just a bomber for example. Sure they can have turrets and extra weaponry to deal with attacking aircraft defensively, but predominantly they are not intended for the air-to-air role. Helicopters work best in a co-operative nature rather than directly fighting each other.
Taking into account most of the suggestions for PvP and how Helicopters are best used (Both IRL and making the most effective use in game), work was directed onto a bespoke PvE that better suits Helicopter gameplay and the environment that would be most effective for them to be used to their maximum potential. So it is directly because of that feedback that this development was even imitated in the first place. A PvE mode presents its own challenges and technical limitations, but work on the mode is ongoing and has taken into account the majority of feedback and issues players had with PvP.