Prevent WWI, WWII, and the Cold War
With a peaceful 20th century, while maintaining the European Empires, greatly slows down technological and social progress.
"slightly slows dow technological in some fields and slow a bit social progress".
Yes,i agree.
But also without WW-II only, i think that decolonization is much more slow (some colonies were very rich of petroleum and raw materials,think to Libia,think to many African countries,and others could not be let go for political reasons..more, without WW-II and cold war the majority of European public opinions don't give a damn of fierce repressions of natives).
In this timeline a relative isolationist America keep the "Jim Crow laws" much longer,probably until the end of XX century or the early XXI.
Obviously American society is quite different from OTL.
No WW-II (and cold war) is not post war boom,no Gj bills,no great corporations of military-industrial system in 50s and 60s,no baby boom and counterculture of 60s, no politically correctness.