Yeah this is the sort of rigid selection deontology I was talking about when I made the post. It's nuts. I actually had you in mind when I made it.
For starters, bowlers are not robots. 18 Johnson overs, 18 Harris overs and 8 Watson overs would work better in this side than 22 Johnson overs and 22 Harris overs, despite the fact that Watson isn't as good a bowler as Johnson or Harris. This is true even when the conditions don't particularly suit Watson. It's not only just true that less tired bowlers bowl better, but also that fresh bowlers bowl better when they're not worried about getting tired. Moreover, Watson plays a completely different role; there are some circumstances where I'm sure Clarke would legitimately prefer Watson on to Johnson regardless of trying to manage Johnson.
But I'm not even trying to make a case for always picking an allrounder, or even necessarily for picking one here. I do believe Australia should pick one here, but I might be wrong. What I'm not wrong about is the fact that your position descends into complete absurdities if taken seriously. I mean, lets take what you straight up said in the first sentence of your post and 'test' it.
What if we really do take this seriously as an objective rule that can solve the problems faced by selectors and decades, and just apply it without thinking to every situation? Lets take a hypothetical player who is the seventh best batsman and the eighth best bowler in the country. He's almost as good as the sixth best batsman -- in fact he's eerily similar to the point where on average he'd only score one less run every 47 innings. From what you've said, you just wouldn't pick this guy. Not good enough to play as one of the bowlers; not one of the best six batsmen. Everyone else in the selection room would be picking him but you'd be stamping your feet, holding up the CaptainGrumpy Selection Bible that clearly states thou shalt not select players who aren't among the best six batsmen or best four bowlers unless as wicket keeper, and warning of coming apocalypse from the angry cricket gods.
From that I can only conclude that your goal is not to win or save as many Tests as possible. Your goal must be something else.. appeasing the cricketing gods, honouring an arrangement you made, creating some sort of employment pseduo-meritocracy where combined skill was considered cheating, adhering to the wishes of constituents from the cult you're apart of, or something else I can't even fathom. Anyone who was actually interested in winning would realise that your one-size-fits-all rule isn't actually conducive to it, because cricket wasn't designed in such a way where team balance was rigid.
Or maybe.. just maybe, you would pick that player. But then you'd be admitting that team balance was not rigid at all, and in fact a trade-off that you usually just took one side on, therefore requiring you to actually have to justify your position on a case by case basis... and that's not nearly as fun as making sweeping general statements that descend into absurdity when applied.