As the back-ups went corrupt as well, there is no conventional way to recover the posts and threads. So the chances that the old posts and threads can be recovered are almost zero.
However there is a very small chance that there might be an dedicated back-up on another location, for instance done for one of of the upgrades. There are still people who are looking for these.
In 2005 we had a similar situation, when we had to change servers. We had to abandon all old data as well and started from scratch.
We are still here and most knowledge is still between our ears

.
What we did in 2005 can be done again!
Huub