There have been many significant wars in the last few decades, including many proxy battles between nuclear powers. So I'd say your thesis is disproven right now without waiting.
None of them have been very significant, though, compared to WWI and WWII. None of them have been more than a schoolyard rock fight in comparison. (Not taking away from the horrors experienced by those countries involved, of course, just pointing out that nothing so horrific has happened on a truly global basis.)
What has changed for the worse, though, is the duration of the wars. For the US at least, WWII was a brief engagement compared to the time spent in Vietnam, Afghanistan and Iraq. Regional wars are basically hobbies for wealthy, idle nations.
