There should be an option to reacquire missions, even if it means restarting the quest chain from the beginning.
Sometimes, when abandoning a heroic or FP, people accidentally click the confirm button on the warning pop-up as the warning pop-up box is very similar to the initial abandon mission confirmation pop-up to those who are fast clickers, and some people want to abandon a mission just to restart a conversation and choose a different response option.
I know that abandoning a mission clears out all response history in prior conversations related to the mission so I understand that it would be difficult to allow players to pick up right where they abandoned it, but it shouldn't be hard to give players access to a mission chain starting from the beginning.
Or, maybe, don't clear the data when a mission is abandoned and instead keep a running log of all completed and abandoned missions in the mission log, maybe a new tab labeled "Mission History" or similar, and give a "Reactivate" option for abandoned missions, excluding heroics and FPs since they're daily anyway.