Long post, have a Tl,dr: Potions and need to be adjusted and made to be useful again at the end game stage.
Potions stopped healing for a mathematically consistent amount by the end of Heavensward.
When you look at the end of ARR, an X-Potion HQ would heal you for 1,300 HP as a non-tank role, when you would have around 5,500 HP. That is only 350 HP off from what 30% of 5,500 HP would be, but that can be overlooked due to iLv being between 115-130 which would give a slight variation of HP.
By level 60, however, the potions are no longer consistent with the math.
This is counting iLv 270 gear, as in ARR potions kept up with the highest ilv gear, which was 130. By ilv270, any non-tank role is looking at over 20,000 HP. And the highest potion craftable? A Max-Potion, going at a rate of 24/30% of HP Restored with a cap of 1,920/2,400 HP. That would be a little more than 10% of any non-tank role; which is a far cry from 30% of your HP.
By level 70, it becomes even more inconsistent. At ilv290, which is a very conservative baseline as it is your Relic ilv, you are looking at over 25,000 HP to any non-tank role.
By ilv380 you start seeing any non-tank role having over 40,000 HP and on the far side you see Warriors reaching 80,000 HP without breaking a sweat.
The highest potion available? The Super-Potion. Which restores a whopping 12/15% of your HP at a 4,800/6,000 HP Maximum. That is the maximum amount of HP healed is what we should have seen at the end of Heavensward potions. Now remember that this came out in the middle of Stormblood.
Mathematically speaking, 6,000 HP is indeed 15% of 40,000 HP. However, that is a far cry from the 40% HP restoration that the lowest tier Potion gives. By lowering the HP% healed in that way you made it so the items become practically useless. Even more so when you take into the account of the fact recast times are going up on potions.
Now, most jobs have a few ways to gain HP back in a rotation basis between Second Wind, Bloodbath and whichever class ability they may have. But Second Wind is on a 2 min recast, and Bloodbath is a minute and a half. Potions are needed for clutch healing, and to do that they need to heal for a halfway decent amount.
Because not only are they -not- healing you in a consistent manner with the lower level variants, they are also a consumable. Which means it costs gil. Either in a straightforward way of you buying said potions, or in the time=money way, along with potential profits of selling the ingredients.
The main problem I have is that in ARR potions were necessary -and- healed for a useful amount. Now at the end of Stormblood they're still necessary, but aren't nearly as useful. Hopefully potions will get a much needed fix, as they are a nice feature that help out especially with all the high end single man instances that have been popping up.
The whole "You don't need potions when you have a healer" argument falls apart right there.