I've wondered this too. It seems like there is a solid argument that they could act as a harmful invasive species and damage or destroy ecosystems which they are introduced to. There is precedent for this in hippos which were introduced to South America.

However, these rare and unique megafauna are so valuable (imo), that the potential damage they could cause ecosystems we introduce them to may be outweighed by their increased resilience. The death of all wild elephant populations would be a massive blow to biology, animal psychology, and natural beauty; I wonder if we should think more about prioritizing their survival over other ecological considerations.

I'm absolutely not an expert in this area at all, just thoughts of a layman.

Shrinking habitat. You need a lot of room for a population of elephants and there are rare endemic trees and animals yet inhabiting those areas.