When I worked in research we looked at the same figures. We had pretty good and reliable data for a couple of aircraft, including very detailed OEW figures that took into account operational items and stuff.
Result was equally inconclusive. There was no appreciable weight benefit over history. We have identified the following reasons:
- I know it is often said, but seats: you simply cannot find a 100% comparable layout. Using floor area as metric may make more sense, but may also not.
- You never find two equal aircraft: some may have same capacity, but widely different range or field performance. A real 1-on-1 comparison is never possible. That is because aircraft are products of their time and technology, and one would be an idiot to build a 1965-aircraft in the 2000s with similar range-payload (see evolution of the B737, for example -200, -500, -600, -6MAX: all same capacity, became best sold model, second best sold model, worst sold model, discontinued model).
- Modern aircraft are not lighter, but they are sometimes much more durable in terms of fatigue lifecycle. They also require less maintenance in terms of inspections.
- Aircraft tend to have always a comparable relationship between empty weight and take-off weight. That is not because it is a physical law, but because only those that have are designed, built and put in service. If some technology makes something lighter, the saved weight usually re-invested (for example in more range).
- Weight is no benefit in itself, it is about cost and performance. Weight may factor in airport charges, but that is about it. And you can always paper an aircraft down. Hence, aircraft are built for minimum cost and the added weight often decreases the cost of operation.
By the way: I like these threads and find them very useful. Just because the result is inconclusive doesn't mean the question and the analysis isn't justified.