Tkanks @Xadhoom for the points.
Evolution is not a game but you do can put the different species into a hierarchical order when it comes to the concept of evolution. Though different criteria may lead to different scoring systems but it doesn't equal the lack of any hierarchy. About the digimon, I think I couldn't clearly mention the point, it is not about a line with no alternative, but for those with multiple branches, bestial ones have a lesser xhance of absorbing attention. Venomvamdemon is the evolved form of vamdemon but he again gains control over his instincts to become Belial Vamdemon. The latter acts and think like a human not a beast and is the final form in the evolution line. Though you might argue that Venomvamdemon is more powerfull in digimon hunters but I can't remember if it evolved from Belial Vamdemon or Vamdemon. Weregarurumon is a beast digimon but the next step in his evolution is Omegamon who is not a beast. When it came to choose between megas for Gommamon, Vikemon got the upper hand and won against the bestial Plesiomon who was the original form for her evolution. In case of Ogudomon and Lucemon's higher forms, they are mindless creatures who just bring destruction and are fitter to this purpose rather than a humanoid mindfull one, they are more suitable for the situation, but you tell me, what is the point for a human angel to loose his humanity to grow more holy features? Do holy power require the owner to becone a beast to get stronger?
Rasielmon is a bipedal humanoid holy angel cat and though her face, body, and limbs represent those of a beast, but her stance and more importantly her clothes makes it a bit hard to consider her as a purely bestial creature. Maybe you've got a point in the case of Raguelmon or Cherubimon, but Rasielmon is a bit hard to classify. On the other hand, even if you consider Rasielmon a beast, she is still another version of Ophanimon as her profile implies and thus she is more connected to the Ophanimon rather than Holydramon. Not to mention that there are obvious elements in Ophanimon's design that are connected to the beasts and can still be connected to Rasielmon design wise.
The less attractive and powerfull looking ones are being replaced or relocated as better designs come into existence in the whole franchise. It goes that far to have Seraphimon evolve to Slash Angemon in heroes in spite of his higher rank in the Archangel hierarchy. I will say one more time, the cast is not bounded by what happened in the past. They can make changes whenever they want and MAKE NEW EXCUSES for that. A lot of them happened in Tri. regarding the rules and rulers of digital world, concept of reborth, and in the case of Gommamon, the mega form. Even in the case of Seraphimon, the design improved a lot from DHT to Frontier, and to Tri. BUT CAN SOMEONE TELL THAT IF PATAMON WEAR A PINK BELT AROUND HIS ABDOMEN, HE EVOLVES TO THE FRONTIER STYLE SERAPHIMON AND THE BARE PATAMON EVOLVES TO TRI. SERAPHIMON? A shift from uglier designs to better ones is not restricted by the events of the past. And Seraphimon is good enough to remain Patamon's mega while having minor changes as newer ideas come into existence.
Your point about both forms being defeated by Cherubimon is completely valid. I think that part of my post was not correct.
Also about the popularity, I think you are right. It is not amatter of deduction too.
Thanks @Theigno. I didn't know that, but it seems my intuition was right, lol. She was not supposed to be a mega digimon in the first place. It would be more reasonable if they stuck to that line instead of messing with everything like this and put themselves into lots of trouble, lol.