A restricted ticket is a contract between passenger and airline to fly on a specific flight at a specific time. If you fail to fulfil your part of the contract, you lose out, unfortunately. That is the essence of a restricted ticket - No use, no comeback! Flexible tickets are sold so that traveller whose schedule may change, can change his flight reservation with ease.
And how would an airline manage all these new miles accounts? Would you expect the airline to maintain 10000's of accounts with a handful of miles held by Mrs & Mrs Twice a Year Passenger?
And how about those airlines that don't have or have done away with their FF
The idea, I believe, would be thoroughly impractical and devalue flexible tickets. Restricted tickets are sold cheaply for one reason - use them, or lose them.