That looks almost identical to the woodpecker holes I see at the top of lots of utility poles. That is an indication (although not for certain) there are bugs inside. My guess is that the tree is dying - and you may never know why.
I would try for a root sucker in a case like this, but that is just my personal preference. I would also do an autopsy on the tree when you cut it down and try to determine what was going on.
Another option would be to make a slanting cut below the decay and try to get a good limb to take over as the main vertical growth. The problem here is getting a tree that big to heal over, and you may end up with the same problem at another place.