Regarding your first point: This is not a bug. Stygimoloch is one of the few creatures that are incubator-exclusive and cannot be found in the wild. Unless Ludia enables players to dart Stygimoloch in the wild, this is the only opportunity to get Stygimoloch DNA for free–without buying special incubators or spending real money. It’s the same with Blue the Velociraptor–without the showcase events or special incubators, we wouldn’t even be able to create her, let alone level her up.
Additionally, each arena that you unlock provides an added chance to receive that arena’s featured creatures, but the DNA you receive is not specific to one arena. Instead, you receive DNA from all the arenas that you’ve unlocked, up to and including your current tier. Prior to update 1.3, other players proved that you don’t have to drop down to the lower arena tiers to harvest T-Rex DNA or whatever it is you’re looking for–you could still get it, albeit not very frequently, from the higher tiers. It just requires patience.
So while I understand your frustration, the 8-hour incubators are functioning exactly as they should be–providing guaranteed Stygimoloch DNA and additional Epic DNA from one other creature. If Ludia chooses to allow players to dart Stygimoloch in the wild, then yes, perhaps then it will be time to change the guaranteed Epic creature. Until then, however, I think Ludia’s doing players a real favor here.