They are larger and build damns and many other differences, so they fill different niches. The idea is to restore the pre-industrial ecosystem as best we can. Maybe their natural land management will improve the quality of the wetlands, or maybe it will cause flooding of nearby human occupied land and cause them to be quietly removed. Maybe they will become prey to wolves and lynx, or maybe they will need to be culled because those predators probably won't be released in London.