Regarding higher speeds in the Pro peloton = I tend to agree with @CycleZen's first sentence stating "cumulative" effects/affects.? And 'fewer race days' has been identified.

Also, I believe the entire peloton is at a higher level of fitness. More Mega-Teams with huge budgets (compared to a decade ago), better athletes with higher salaries/side jobs, 'training at elevation' more often, more specialists primarily targeting either 1 day, 1week, or Grand Tours. Lack of UCI/WADA to be more strict on 'nutrition' and 'medications'. (what gas have some been huffing lately?). Better training at youth levels leads to younger super-stars.

Maybe another slight consideration for stage races are changes in the name of 'rider safety'. Shortened stages due to inclement weather, less hazardous courses (l'Angrilu used to be a goat path, now has mtn pavement), etc.

The Classics and Monuments remain the same, largely.

(apologies if all the above has been mentioned, in addition to stiffer framesets, aero tubes, better tires, lighter wheels, skinsuit fabrics, etc.)