One of the fitness requirements for sevens which coaches look for is repeatability. A speed merchant will often need to accelerate and go flat out only moments after he's just done exactly that. It's less of a requirement in the fifteen man game. Some players naturally have that kind of engine, while others need conditioning.
I don't think anyone doubts that many top players can make the grade at sevens. The argument seems to be mainly about how long it would take to get back to performance levels in the shorter code. Mike Friday says 10 weeks while another coach quoted in today's UK Telegraph thinks two tournaments would do the trick.
The window is probably between the end of the 2015 World Cup and Rio, which raises another issue. If the World Cup succeeds in boosting rugby's profile, is it really helpful if top stars immediately drop out of the fifteen man game at club and international level?
The IRB has said that the sevens World series next season will come under regulation nine. If teams select players, then clubs will be obliged to release them. That's probably more of an issue in the North, where teams have overseas players from most of the countries expected to be in Rio. The World Series also runs from October to May, so every tournament overlaps with part of the season on the North.
The Asahi newspaper covered this (the only one of the major newspaper titles to do so)
The link is all in Japanese, and some of it is behind a paywall, but you might get some small sense out of a translation programme.
It mainly describes the procedure - Japan must submit a proposal by a date in August - but mentions that Singapore has emerged as a major rival. The belief in Japan is that South Africa supports the Singapore proposal while Australia wants Japan. No-one is sure where New Zealand stands.
There is a Japanese rugby forum on this topic in French, with occasional English, here:
I agree with Eddie Jones. From an IRB point of view, and growing the game internationallyy, if Japan wants to be included in Super Rugby, then they should get first shot.
JPR Williams stopped a certain try and kept the Grand Slam alive for Wales.