Interesting discussion. I think some of the seat tube angle decisions had more to do with aesthetics than anything else, i.e., with a desire to make the head and seat tubes approximately parallel, to make the frame "look right."

My 1960s era road bikes (1959 Capo to 1970 Peugeot) all have 72 degree seat tubes, and the 10-years-later Bianchi has 73, as did my 1971 Nishiki Competition. All have equal head and seat tube angles. The Peugeot is disproportionately long in the top tube by at least 2 cm, and the Nishiki was correspondingly short in the top tube. I can comfortably use a stem of typical/average reach on everything else.