Flex characteristics(or lack of) are a big factor in modern MX bikes. To get the same rigidity from steel it would be heavier. I don't think the YZX is in the same league as a bike with an aluminum perimeter frame. The YZ I rode seemed forgiving as steel.
There is no doubt that if the Japanese made the decision to design and build a modern 2stroke in numbers it would be big trouble for the Euros. Look at the early '70s when the quirky but good handling Euro bikes ruled the MX tracks. Jap MX bikes were just stripped and ported dual sport/enduros of the time. Then came the Honda Elsinore. Dirt bikes then were the SXS of today, unfortunately not enough $$ in bikes anymore.