A player going overseas as a young journeyman will be on a fairly small wage compared to what an Ex-Wallaby would be on. Even if a lad goes overseas at 21 and later is selected to represent Samoa (or Tonga/Scotland or whatever) the future Club contracts would still be less than what a recent Ex-Wallaby could earn.
Kind of like going overseas immediately after graduation with a BA degree to get some experience before attempting your masters degree. The BA graduate gets entry level wages that are still better than what they could get back in Sydney, with minor annual increments for experiences in the job, but no major leaps in salary until they get post graduate qualifications.
While overseas they see Australian University Masters graduates swoop in and get massive salary packages far above what they are on as an experienced Graduate with no post gtaduate qualifications. In this scenario, the only way the young bloke can get a Masters qualification (and the big package that comes with) is by returning to Australia. Some people are happy to plug away on a decent wicket overseas, not bothering to take a risk and attempt the Masters programme, because they know that there are better candidates around that will qualify on the masters programme before them, or may even deny them entry into the masters programme entirely.
Is Australian Rugby better or worse off with the likes of Dan Parkes, Keleti Vaingalo, Sam Norton Night and co playing overseas? Some of our overseas based rugby players realise that they do not have any chance of ever making Wallabies (or even Super Contracts), so they stay. Good on them.
The lure of Wallaby Gold which is only available through Super Rugby will continue to keep ambitious lads here, and will attract ambitious lads home after a bit of OE.
Is there anyone overseas that we genuinely can not replace with a better player locally?
Sure we will lose some, just as we lose good teachers, accountants, nurses to foreign lands, but the challenge is for the ARU to keep a nice broad pyramid in the player base to replace the losses with equally talented young'uns.
That is the problem that ARU faces as it offloads the heavy lifting of junior rugby development to a small ( and ever decreasing ) number of Private Schools engaged in a predatory Arms Race.