The "mistake" is that the assets themselves are the source of income. Sell them off, and the income goes away too. It's the equivalent of expecting the parents to use 100% of their income to put their kids into college, which is impossible.
A bunch of stock is a source of income too, but it wouldn't be wrong to use some of it.
If the business is worth enough then selling it can replace all the income you would have ever gotten from it. It's not as simple as "income goes away". The specific numbers make all the difference.
IF I have stock and make $XX,XXX in dividends, how is that different? IF I have own apartments and make $XX,XXX in rent, how is that different?
I think the idea is that Yes, the expectation is for people to make actual sacrifice before they qualify.