This completely ignores the complexity of scale. For one given tweets up until now were immutable there are likely tons of systems with that assumption baked in

This completely ignores the complexity of scale. For one given tweets up until now were immutable there are likely tons of systems with that assumption baked in that you now have to reevaluate/rework. Think caching layers and data storage. Do they need to store an audit history (I assume this requires a few different legal teams in different countries)? How does that scale? The picture is far bigger than the three main variables you mention.