When frequent flyer programs launched award tickets were supposed to be a reward for loyalty, and award customers were expected to be treated better than paying customers. There are often steep change fees on award tickets now, but mileage awards are for the most part still cancellable with points redeposited into an account. In other words even after myriad rule changes over the years, inventory issues aside, they’re still more flexible than paid tickets.
Miles aren’t just an alternative currency used to purchase travel or services from an airline, they are a thank you for your business, and redeeming miles should feel like a reward and not a punishment. Delta got this one right.