If the price of the staple increases dramatically, can't they shift towards eating other crops (that they were previously net producers of)?
Again, those other crops are likely more expensive per calorie. That means there is no mix of consumption that will get them the same amount of calories they previously consumed, which for subsistence farmers will often mean malnutrition if not starvation.
In terms of effort required to grow those crops, or in terms of market price? If they're growing the crops in order to eat them, the market price isn't relevant (except that if those other crops are expensive on the market, they can sell those for a profit, too).