Interesting discussion.
So, here's my theory of inertia. Let's drop a dimension and go to the rubber sheet analogy. A large mass makes a large indentation in the sheet with a smaller body nearby making a small indentation in the "downward" slope of the larger one. As the small body "falls" towards the larger one, its small indentation must propagate through the sheet. The resistance of the sheet to the moving deformation is analogous to the inertia of the small body.
The greater the mass, the larger the indentation, the more resistance to motion.
Bingo. Inertia!

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Does earth plug a hole in Heaven or Heaven plug a hole in Earth? -Peter Gabriel