I see no reason why anyone should expect an airline to waive the cancellation/change fee in case of illness. Even with a Dr's letter.

It has become standard practice in most countries to offer cancellation insurance policies when purchasing deeply discounted tickets. These cover any changes needed due to legitimate illness. I can see no reason for airlines to offer any consideration in waiving the change fee, even for illness since insurance is widely available.

Why should the airline indemnify a passenger? If you want to change, pay the fee. If you might get sick, get insurance. If you don't want to pay change fees or buy insurance, then buy full fare tickets.