I'd say you'll have a good chance to make the IAD connection, but if you don't - depending on how delayed you are - I wouldn't give up on the DCA-DFW option as a backup (which AA should put you on at no cost). DCA is nothing like IAD. With boarding pass in hand, PreCheck, and no checked bags, you can typically get from the front door thru security and to your gate in 15 minutes or less. Have the cab drop you off at the express passenger drop-off (follow the purple signs) which bypass the ticket counters and enter right into the metro station next to the security area (PreCheck is first).