I think it clearly illustrates that the relegation contender from the 6 nation will have an opportunity to stay in the competition through a playoff as Rugby Europe has already implemented in its structures. So if they are good enough they will stay and have another year to build. If you wanted you could even have the 6 nation side playing at home.
Been a few posts since but...
I don't believe play-offs exist to ensure the team finishing top of whatever tier sits beneath is good enough to actually be promoted - more that it's protectionism for the upper tier team in that even if you spend a season getting spanked by better sides, the odds are you'll be more battle-hardened than the team in the lower tier so have a natural advantage.
Or for example, looking the (soon to be defunct) English system for getting into the Permiership - the play-off is between two sides in the Championship so it doesn't apply.
That aside, even if the side seeking promotion can win in a play-off I don't think it invalidates the point - just because you can sneak past the worst side in the league (or whatever you want to call it) above, doesn't mean you're any less cannon fodder the next season.
I dare say it's different for domestic sides as they can recruit based on their elevated league status - doesn't really apply for national sides.
As I said, I don't know the answers but I think an annual play-off system is flawed, particularly at international level.
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The weekends are booked 3 or 4 years in advance but not the teams you're going to play, give home advantage to the current 6 nations side and have it as their minnow bashing match. If the side seeking promotion isn't good enough to beat the worst 6 nations side away from home they'll just go straight back down anyway!
At this level. barring a miracle, they're going straight back down regardless imho.
The money and time required to build a competitive side at international level means that one season isn't enough.
Unless you have some time in tier 1 then you have little hope of getting enough money to provide the required funding so I just don't see how they can plan long-term in a way required for success.