Nerfing something and having something take it place is fine and it keeps the game refresh. Instead of playing the excat same strategy over and over.
Since these monsters werent free a compensation system could be implemented i suggested before opening a shop were you can sell nerfed monsters and replace them by a monster of your choice.
My idea was
Create a store with all the legendries monsters in the game.
Normal monsters sell for orange gems
Limited time monsters sell for red gems
Legendary potions are returned.
You can only sell nerfed monsters and replace them. This system allows devs to have more control over the game balance and us as users dont suffer from nerfs as we will be able to replace the monster with a monster of our choice.