>It doesn't matter if it costs me $4000 in rent, groceries, transportation, etc., to make $6000, I'm getting taxed on that full $6000.
So if I spend $5000 on groceries because I'm eating wagyu steak and lobster everyday, is that a fair "expense" too? You might retort that's obviously a luxury and there should be some baseline that's tax free, but then you're just describing the standard deduction.
>but as long as the value of investment is positive, that is, outpacing inflation, it makes zero sense to let your money languish in a Scrooge McDuck pile rather than get some value out of it.
...or they take their money elsewhere instead.
Businesses don't pay extra tax because they chose to met their needs at a greater-than-minimum cost. If my boss buys some reagents from Dave for $300 bucks instead of Bill's $200 offer, the company's not paying taxes based on a hypothetical $200 cost basis.
If we want to tax "luxury" expenses, we have excise taxes for that.