*Snip*
Hrrrm.
I have no doubt that the French Revolution, and a general rebellion against nobility/aristocracy in Europe, was tributary to fashion changes. But I doubt that the French Revolution was the source.
Look at Oscar Wilde, for instance. Dandies were condemned for snobbery and effeminate clothing, but nevertheless there was still a cultural construct capable of maintaining their lifestyle more than a decade after the French revolution.
I'd say we ought to look broader than one event. Revolution is a mindset, not an event, to paraphrase Jim Butcher.
The French revolution, which targeted the trappings of a particular way of life, came at the same time as the Industrial Revolution. Things, including clothes, were made en masse - and capitalists love shaving pennies off the cost of production. Goodbye lace, embroidery, decoration. These were all time consuming, produced by hand and expensive.
Another significant revolution was spiritual. The Reformation, begun in the early sixteenth century, established criticism of ornamentation and boisterous colour. This extended beyond criticism of ornate Catholic cathedrals and into critique of individual dress. Good works defined man, and the rich man could not make it through the gates of heaven. Sobriety and charity were not indicated by boisterous clothing.
The paintings of the Dutch Republic show spiritual men in sombre black.
The third revolution, and I'd argue the most significant, was in travel. Globalism established a mono-culture dominated by colonising powers such as the American and European nations. Mass-production of drab factory clothing would not have mattered were it restricted to a single nation, but it wasn't. Cheap clothing dominated the global market, undercutting clothiers everywhere.
There were more contributory factors, but I've said enough to illustrate that the French revolution was a symptom of broader societal changes which influenced fashion and - besides - that other events and even mentalites contributed to today's bland fashion for men.