McKellar is interesting, assuming all coaches aspire to be a Test coach I'd say the tree is generally Test assistant -> Super/Club coach -> Test head coach so he went from a stable job at the Brumbies to Wallabies assistant purely for succession planning but now that's gone for the next 5 years. Larkham is back at the Brumbies. Does he want to be an assistant for the next 5 years (does Eddie even want him??)
The time as a test assistant definitely helps his case down the road, but as it stands he could do with more head coaching experience before taking the role on. Test Assistant -> Head Coach only seems to work when the side is going well anyway, it's not a position we've been in to pull that transition off from for some time.
I do agree he won't look to hang around as an assistant for 5 years though, that said there will probably be a couple of super rugby gigs open by next year (Rebels and Reds look likely) if he wants to to stay in Australia. Should be plenty of options open for him OS as well, I just hope RA don't burn the bridge there if he does take one of them.
The other part to consider will be what actually happens with the Wallaroos program. It sounds like Eddie is overseeing in a Director of Rugby type roll, so I imagine there's still a head coach gig that will probably see increasing investment over that 5 year period. Not sure if this is something McKellar would be interested in, but it'd be a good place to build that resume while remaining close by.