Attempts to introduce species have turned decidedly sour in the past... if introducted in ecosystems where they do not belong. Reintroducting a locally extinct species is a different case.
Big felines are very well known animals. Present in all zoos of the world and reproducted in captivity since many years ago. There are entire books published about big feline surgery for example.
It is a conception because it is not the previous state you're returning to.
This is a false problem. The goal is not to achieve a theorical ideal previous state, is to restaurate a missing node to be able to reach a range of valid stable states. Is the same difference as "to reconnect a route bombed in 1940 to drive from a to b" or "to try to frozen the area in 1940". Nobody is talking about to living in caves again and working three hours each day.
If there's an insect that affected a plant... causing an imbalance that has not been allowed for.
All imbalances are "allowed" in fact, there is not a nature police, but your imbalance will never occur. Hervibores will just start eating other species of plants or migrate and quit the area followed by predators until the plant species recover. Mature ecosystems are buffered and can deal with known changes.
Attempts to introduce species that we thought we understood well have turned decidedly sour in the past, introducing species we know relatively nothing about is unlikely to go well.