It would be totally ineffective to replace the assistant coaches while retaining Cheika. Either Cheika takes the responsibility and is sacked or resigns, or all of them go. Keping Cheika with a hotch potch of new assistants would be about the worst outcome imaginable.
There will be very little opportunity for any new coach to stamp his authority and personality on the team with only four (?) more tests between now and the RWC and only two training camps in which to do it. But something surely can be done to improve the situation, by concentrating on implementing a new defensive structure that doesn't require constant movement in the backline, reducing the number of set plays to confuse the attacking systems, concentrating on individual skills with a large part of the training sessions being set aside for this, and re-establishing the set piece as a strength. Couple those issues with selecting players who can perform the roles of the positions they are in, including an emphasis on the defensive aspects, and someone with a bit of background in coaching could help to pull the Wallabies out of the current pile of shit they're in. That person certainly isn't Michael Cheika.