While BC for “convenience” is not permitted, many Rabbonim who are in touch with current realities do allow “spacing” and BC for financial reasons. The explanation being that women today are not physically strong enough to have children too close together and “spacing” may be a medical necessity. Also financial crisis can affect a person’s physical health, so preventing a nervous breakdown is a medical necessity. Nursing is not a reliable form of birth control, though it does reduce the chances. Nursing purely for purposes of preventing pregnancy is no more permitted than taking the pill, and I am familiar with cases where people malnourished their infants to nurse them exclusively for two years just to space the future children. They baby cried the whole day from hunger, but the parents would not supplement with formula or baby food because it would reduce the contraceptive effectiveness.
The bottom line is that BC is bot to be taken lightly, but is permitted in many circumstances. Each person should ask a Rav who is both qualified and knows their personal situation.