A single commerce degree is very useful because you can do a double major-accounting/finance, accounting/management. As long as you do the mandatory CPA/CA accounting subjects, when you graduate you can train to get your CPA/CA. All degrees can give you a certain amount of wealth, but even if you do commerce, the graduate pay for accountants isn't that fantastic, about 45k-50k. In contrast, the graduate pay for dentistry is extremely high- I think it's mid $60k. So for the superficial 'bros' who want to get rich, dentistry is definitely a better option than commerce. (provided they can get in)
Not everyone who does a commerce degree wants to do a double degree, why waste two years of extra fees when you can graduate at 21 y.os? Commerce is useful depending on what major you do, if you do accounting and get mediocre marks, that is still acceptable. But if you do management and get crappy marks, then commerce is not as useful.
Doing law without a minimum amount of passion is really hard because you would not be motivated to study hard as the years pass. Also, not everyone can get into law (unless you go to the lower ATAR universities), so doing commerce with law should not be a do or die attitude.