We've spent the last 10 years replacing players as well. Doesn't seem to have helped any more than replacing the managers has.
It's obviously a possibility that both the players and the manager can be a problem at the same time. And based on what we are seeing it's not just a possibility, it's a strong probability. In which case, why allow a failing manager to continue training and managing the club which will likely result in another 12 months of failure, training a system that isn't working, and have influence on the transfer targets?
With the new football structure coming in I would hope that they already have a decent idea what the priority changes need to be, so a new manager will come into the club already being told that certain options will be changing. Sure they won't have as good a hands-on idea as ETH does, but that could perhaps be a good thing if ETH has formed attachments to players who aren't good enough.