Alternatively, younger guys may choose to go earlier.
Let's say Kepu and Slipper head overseas next year as the 2 best Wallaby props.
A young guy may just decide there's no benefit to staying here and playing for the spot that would have been left vacant by these guys - they are still eligible and probably will continue to be top choice for at least another year or two.
May as well go and make some money in France or Japan for a year or two.
The sword cuts both ways.
I don't see this change influencing this. If you're a young player and you're clearly behind Slipper and Kepu what does it matter where they are playing (now that the rule has changed)? They're going to get picked ahead of you anyway.
There are four props in every matchday squad and with Kepu leaving you'd expect another top up contract to be available. There's still substantial financial reward available for players staying in Australia together with the chance of securing a regular Wallaby spot (even if it ends up being on the bench).
I think the Wallabies will remain a team largely made up of locally based players. It will very much be the exception for foreign based veterans to be selected in the team. As others have said, European players only get released for test windows so they miss out on training camps etc. Due to those drawbacks I think in any close run situation, the local player will get picked.