Well if some HQ mats are dungeon drops then there will always be an incentive to go back to those dungeons.

1st of all, the drop will not be every run, so you'll have to run it multiple times to get a HQ mat.

2nd, More than 1 person in the LS might need the HQ mat for their HQ synth and it might be used in multiple recipes so that will make that you have to run the dungeon/zone multiple times over and over.

3rd, More than 1 HQ mat from several dungeons/zones/HNMs might be required so in order to get all the HQ ingredients you will need a LOT of runs and even more so to craft all HQ gears for everyone in the LS.

4th, when the cap is raised and new content is added, there will still be incentive to run (even tho the content might be trivial) the lower tier dungeons/zones/hnms because higher recipes will require the lower level HQ craft, so you'll need to run it again to acquire such lower HQ mats to make lower HQ gear to craft higher end HQ gears/items.