I don't think England can beat Argentina at the moment.
Well we will see in a few weeks given that they are playing each other in their first fixture in the world cup. Sure Argentina could beat England, but it is a long way from a forgone conclusion - certain the betting odds don't suggest that with England are 4/6 favourites for the match with Argentina 7/4 against.
But your claim was that it was a 'formality' that a 3 placed team in the rugby championship would beat a 4th ranked team in the 6 nations - the implication being that the rugby championship team will always beat the 6 nations side. That is simply non-sense, as the betting odds for the upcoming England v Argentina match suggests.
But there is a broader point - what I want to achieve with my proposal is to keep an interest for as many teams for as many rounds as possible in both the rugby championship and the 6 nations. Under my proposal there is an incentive to finish 2nd rather than 3rd in the rugby championship as the former provides automatic qualification (a real formality) into my proposed new tournament, while the latter provides the uncertainly of having to go into a play off. Similarly in the 6 nations - there is incentive to finish 3rd rather than 4th as the former provides automatic qualification while the latter only result in playoff. But there is also incentive to finish 4th (where the opportunity to qualify for the new tournament via playoffs exists), rather than 5th where there is no chance.
Simply allowing the top three from both tournaments to qualify significantly reduces 'skin in the game' to try to finish one place higher.