As this is a sort of metahistorical or historiographical question about periodicity I know that there are other people on the sub who might have more to say but 1. It depends a lot on where you're talking about and 2. The periodization of "modern" is not so black and white as you're suggesting.

So to start with the second point first, the distinction is not between "modern" and "pre-modern" as you've laid out but is more typically between "modern" and "early modern".

I think also implicit in your question is not just one of historical periodicity but I suspect also a value judgement about modern=modernity=A Good Thing. Most historians who make that periodization I think would actively argue against that notion. Insofar as events like the American Revolution or the French Revolution led to major social changes, there's a reason why one of the major historical journals of sociology is called "continuity and change." Both revolutions involved significant societal change, but also continuity from the previous period. Thomas Jefferson, ambassador to France at the start of the French Revolution, didn't wake up the day the bastille fell and go "Aha! I am a modern man now!"

To return to the first part, the American Revolution or the French Revolution may or may not have relevance to periodization depending on where a historian is writing about. So, an historian of the "modern" United States might focus his periodization around the American Revolution (if anything, the very concept of a "modern" United States for such a young country means that I haven't seen this periodization very often for either the 13 colonies or the United States, though I can't claim an expertise on the topic.) His European counterpart on the other hand might focus her periodization on France.

Other parts of the world have other choices.

In the Arab Middle East, the stereotypical choice for the "modern history" periodization isn't the French Revolution, but the French invasion of Egypt in 1798.

Location also changes what preceded or succeeded each period. So while historians of Europe might date the end of the "Late Medieval Period" and the start of the "Early Modern Period" to some point around 1500, historians of Persia like David Morgan can comfortable write a book with a title like "Medieval Persia 1040-1797".

I think for other viewpoints and discussion, about a year ago we had a really great meta post on periodization and regionalization that may be of interest: https://www.reddit.com/r/AskHistorians/comments/4y1739/rules_roundtable_17_periodization_and/?utm_content=title&utm_medium=front&utm_source=reddit&utm_name=AskHistorians