I agree with you to a degree, but he has not helped create national dominance on his own. Spain won the Euros before he even joined Barca's first team. The dominance was already established. Same with Germany, they had a great young squad who were one of the best teams in the world and Spain's golden generation ending opened the door for them in 2014.
And with England they overhauled there systems long before Pep, it led to them doing incredibly well in youth tournaments. Then of course Pep and the other top managers helped develop that talent, mainly Pochetino that is.