I'd probably err on the side of caution if I were you. Dingo was supposed to be Australian rugby's savour, followed by Link, then Cheik-mate. Unyielding optimism and lofty expectations from the rugby public at the start of each tenure followed by resignation of the status quo. The trend only continued. New coach isn't no silver-bullet fix, improvement must come from within and fortunately we are starting to see the rewards of Castle's investment in youth programmes and talent identification & retainment. Aussie U20's did well in 2019, which bodes well for the future.
Rennie ain't a miracle worker. My advice is to be realistic about results against the top rugby nations for the short-term.
Look for an improvement in the on-field structures/tactics & attitude from the Wallabies over the next couple of years, emphasis on good foundations & structures layed-down rather than the results in the short-term. Ideally what's put in place by Rennie and the coaches over the next few years should be intended towards having the team achieve consistent results in the long-term, but 4 years from now. Most likely post 2023, because this a very young squad, that will realistically only peak the following WC cycle.
Just don't expect them to suddenly become world-beaters overnight, because at the end of the day you're still playing the 1st and 2nd ranked sides in the world respectively, regularly. Rennie can't be made a scapegoat for not getting a non-rugby nation (a sport ranked behind ballroom dancing) not defeating rugby obsessed strongholds (NZ, SA) on a regular basis.