If a 10' fin had 5' of mounting width and a lot of structure, you would (or should) feel pretty comfortable. With 6" mounting surface it really doesn't matter what the structure is. Now take that same 10', add 5' up into what amounts to a daggerboard trunk, and you get the 5' of mounting and a way to provide the structure - all without changing the hydrodynamic shape at all. The bending load in the keel fin at its root is the same (and is usually not an issue), but the max stress in any of the hull is a tiny fraction, it could be held safely by some very small bolts. But this is not how it is typically done.