Depending on the tree and local conditions it can take tens or hundreds of years for them to decay. That’s why old growth forests take so long to restore, they require several generations of dead trees in different states of decay. This allows everything to fungi and insects get established.

They definitely shouldn’t have removed dead trees. Worst case scenario if they were ugly and someone politically powerful wanted them gone they should have been broken up or ground into large mulch.

I was once at a conservation event with the Wye and Usk foundation and the Wild trout Trust. WUF presented first on removing obstructions, then WTT presented on large woody debris! WUF, however, was rich and could push out other people and groups and push their vision. Another group were working on a novel way to stock salmon (semi natural rearing, pioneered successfuly on the Tyne) and WUF campaigned successfully to have it banned! It was a lesson to me as a young man that not all conservation groups were net good. The rod salmon catch on the Wye was 188 last year, down from 6,000 in the 1960s.