Maybe saying there should be six nations games in America is taking it too far, because of the tradition etc as you say. But World Rugby is correct to want to promote the game in America.
Not only to grow the game but also to compete with NFL, since NFL is already targeting rugby countries for their growth, using this same idea.
Just yesterday the San Francisco 49ers played the Los Angeles Rams in Melbourne, Australia, in front of a sold out stadium.
Rugby will either compete and grow or go into terminal decline.
With invest, I take it you may mean that thing where you spend and hope for a return. But these USA fixtures can be instant wins. The Springboks vs All Blacks game in Baltimore will be in front of more than 62,300, making a heap more more money for both the Springboks and All Blacks than any of the ones played in South Africa. It will be the most-attended single rugby match in North American history - not only making money but also showcasing the game.
Nations Cup games would be great for this idea as well - not only in America but also in other places you mentioned such as Madrid