The Peugeot PX-10 is an awesome 1950s racing bike that lived on with relatively minor changes well into the 1970s. C. 1973 Peugeot began selling these with more upright, stiffer frame geometry before dialing that back a touch to maybe 73 degree parallel angles and clearances that reflected what was then fashionable. All the while they kept sponsoring racing teams including the '75 and '77 Tour de France winner Bernard Thevenet - who admittedly rode a PY-10, but that is simply a more refined PX-10.

A '60s example like the OP's is designed for long hard stages over roads that weren't buttery smooth asphalt, but could include dirt and cobblestones. Typically the tubulars would be fatter (28-32mm) because good fat tubular tires are fast and comfy at the same time. There are clearances for mudguards because these bikes were ridden year round, when racers would bundle up and head out for long training rides in bad weather. A stock PX-10 in 1965 would have weighed less than a comparable Campagnolo-equipped bike and would provided arguably smoother shifting. The metric gauge 531 tubing would have provided a springy ride over poor road surfaces.

Referring to a PX-10 as a clunker sounds like a comment from someone living under a bridge waiting for unwary billy goats, but it may simply reflect not thinking through the biggest single influence on bicycle design changes over the last 50 years - affluence. The whole idea of maintaining a fleet of dedicated bikes for specific cycling experiences is relatively recent. Lots of cyclists used to have a good bike and maybe a couple of wheelsets, riding clinchers for commuting, touring and club runs and switching to sprints (tubulars) for time trials and racing.

As noted earlier, this one is NOT all original - the derailleurs, wheelset and brake levers are all later products. It's a lovely bike, and in its time was a race-winning bike - lots of amateur racers were won on these machines as well! - and judging its raceworthiness by the standards of later hyper-specialized machines only capable of being ridden on glassy smooth asphalt without shaking fillings out is missing the point.