Not on board with this line of reasoning. In 2005, most of the top players in pro pool were still American. Entering the 21st century, only a handful of Europeans were full-time players of American pool. Among them were Ortmann, Souquet, Feijen, Lely, Chamat, and Immonen, and nearly none of the others traveled to America to compete. In all the major pool events, the fields before 2000 featured very few Europeans.
The watershed moment came in 2000 when Matchroom, which produced only the Mosconi Cup and the World Pool Masters at the time, made its first ever move into large-scale tournament pool, producing the World 9ball in Cardiff for 2000-2003. Although the Euro-tour had existed since 1992, for many Europeans, and I have spoken with some of them over the years, this is when American pool came into full view among European cueists. In 2004, the World 9ball moved to Asia, but Europe's interest in American pool had been piqued, and from 2005-15, the number of Europeans playing American pool skyrocketed. From 2015-25, the game became increasingly global, and there are now great players in every corner of the globe.
Americans dominated at American pool prior to 2005, to some extent, because they were the ones playing it. While there is no denying that pocket size figures in the equation, attributing America's fall from grace to the tightening of the pockets is missing the boat. Everyone plays 9ball now, and the American domination seen prior to 2005 may never be seen again.