I recently got over 50 legendaries. Looking through my list of them I can say that there are 13 I’ve never ever used in a team other than for Whale’s Treasure. Does this mean those 13 are pointless?? No, it just means that there are better options in my other 40 or I don’t have a strong combination of monsters to play with those 13.
There are stronger and weaker legendaries along with a general inflation to their performance in newer ones. Many of the older legendaries which got outclassed or outdated were given buffs so they have some niche use or do something unique but we can’t expect every single one to be viable. Any of the non-limited monsters are basically subject to be outclassed at any point, because the limited monsters are what drives the game forward and brings income for the developers.
So when there’s any reasonable argument for a niche use or unique aspect of a non-limited legendary then I don’t think we can expect them to get a buff. We certainly shouldn’t argue amongst ourselves over it!
Nebel/Cosmo are DR duplicators. That’s strong in itself! They’re not viable in top tier PvP or for experienced players with a large selection of legendaries but they have their unique aspect and are even very effective in a much loved part of the game… UC.
Cryo/Sancta are the only monsters with stun revenge that can duplicate themselves. They have high damage, a swift attack boost and assisted attacks so can do surprisingly reasonable damage despite their obvious lack of strategy. Any clever strategy with good legendaries outclasses them but they’re not completely useless.
Rexo/Kami are another old duo who come up a lot. Now these guys I can actually get behind. They have no unique passives or moves, just a simple “get a kill with quite bad tools” then bloodfury. Bloodfury is exceptionally strong so this is probably why they haven’t got a huge buff, especially because they were the only original non-limited legendaries with bloodfury (ignoring Emeraldeus which is awesome and has been re-branded limited). However, they have nothing unique to offer and there’s no niche place where people would think to use them because a huge number of sweepers simply outclass them. These probably need a buff of some sort.
I hope you can all see where I’m coming from 
