You said that "modern frames have a much more aggressive head tube angle", but comparing old and new geometry charts disproves that. You said that straight forks are raked less than we might think, but geometry charts show similar amounts of rake/offset on 1980s curved forks and current straight ones.

If you cherry pick one old bike and one new bike, like the 58 cm 1984 Trek 660 and current Madone referenced above, you'll find some minor differences. In this case, the head tube angle and fork offset result in a 4 mm difference in trail. You'd find similar variation among bikes of the same age, whether they're 30 years old or brand new.

So what is it about geometry that has changed drastically?