This "issue" is real, but, n the other hand, all other players in the passing years had to farm these mobs with exact this low respawn rate. So i think it might que quite unfair to change/reduce the respawn time for players who farms these mobs now. It is time consuming, yes, and i wished the respawn would be better, but... from the beginning, not now.
You could consider to analyze this for the zones, which are not yet changed to the new card system and make adjustments BEFORE you implement the new system, to have fair chance for everybody.
In this game, not all is really standard as it should be, or as you would assume it should be....