I wonder if a lot of those countries would do better to declare bankruptcy and not pay those debts.

If by country you mean the people who live there, yes.

If you mean whatever oligarchy is nominally in charge, well, the bankers can make sure they’re taken care of as long as the interest payments keep rolling in.

Iceland did this? And it worked beautifully.

and you would have a situation like recently in Sri Lanka where they defaulted on loans, and cannot obtain necessary resources like oil to run their economy.

IMF loans are bandaids to stop the bleeding, but the cure depends on the disease. If the country does not cure the disease, the bandaid will not help after a while.