Imo, just getting one 5850 to later crossfire doesn't really work... why? because you will feel that the 5850 is more than capable when you get it and when it starts struggling you want to upgrade to a new card anyway.
This could be the case, but it can also be that the 5080 will become so unbelievabily cheap that the user decides to get another for a 30-40% preformance boost.