I don't think it's weak - it's the unions playing their trump card. At the very least, players will know what they're signing up to, at best (for the unions) it will force R360 to negotiate in relation to Test availability.
Most of those unions already exclude foreign based players. Even South Africa doesn't pick that many from outside its domestic teams. Plenty of top players still move in their peak.
But there was never going to be a huge exodus before the world cup anyway - most top players are already signed until then. But what happens if over 50% of the Springboks, All Blacks, England and Wallabies world cup squads sign on to R360 for 2028?
I suspect this will happen straight away in the womens game. If almost all of the top 100 female players in the world sign up, do the unions still ban them from test selection? They can, but then test rugby will clearly not be the pinnacle of the womens game. It'd be like tennis in the 1950's and 60's - Wimbledon still existed as a tournament but it was never won by the best players in the world because they played on the professional circuit and were banned from it.