Good morning,
I certainly agree that older bikes can be great to ride, although I wouldn't want go back as far as the
https://www.cyclechat.net/threads/bsa-tour-de-france.239856/
BSA restoration thread.
I had a similar bike as a youth, a Raleigh Shadow, same frame, brakes and wheels but 5 speed, this was during the period that Raleigh, BSA, Sun, Carlton etc were all just different paint jobs.
For years I have avoided progress and mostly ride a 531 steel/downtube shifters era machine, but 6 months ago I was tempted by a lonely looking used full carbon/Di2 jobbie.
I really wanted to see what I was missing, I now happily swap between the two, the lighter frame is a noticeable improvement but I am equally happy with the steel frame, but Di2 is a genuine, real upgrade, for me at least.
The ease of changing gear for just a few seconds as I go down a dip and then back up again means that I ride faster but with less rests.
Although I can't imagine it ever being Shimano's intention, changing onto the small chainring when approaching a busy roundabout and then changing back up even before leaving the roundabout is a real nice way to negotiate it without being knocked off by impatient cars. :-)
The sad thing is that the only real improvement, for my type of riding, is the also the most expensive, carbon frames and steel frames are pretty much on par price wise. Clipless and toe clip pedal are also similar prices assuming you believe that clipless is an improvement. :-)
Di2 requires at least Ultegra components, as there is no sign of Di2 105. :-(
As a piece of useless info; a couple of years ago I put a new Ultegra front mech on the steel bike, when they introduced the long pivot arm, and it is a really fast change with downtube shifters and may be worth considering for your Peugoet if you use both chain rings often.
Bye
Ian