All hypothetical but given the choice of England winning the World Cup or us winning on the opening day of the season it would certainly have been the tournament success. In fact the two achievements are not comparable imo. That would have created an immense buzz akin to if not exceeding winning the fa cup.
I do agree that international football in general outside the tournaments doesn't generate much interest for me, in fact I find the breaks irritating as they interrupt the domestic game. However during a tournament it unites the whole country and brings supporters from all clubs together which is a positive thing.