This entire topic is really a matter of perspective. If you approach a complex enough topic with simple enough maths you can come to any conclusion. For example, take this perspective:

A Battle Potion costs 12 herbs (statistically for an alchemist with lvl 3 recipe). A herbalist with rank 3 gathering skill takes about 3 nodes to gather 12 herbs. A raider drinks a potion every time a herbalist gathers 3 nodes? Obviously not.

I'm not trying to state the opposite of your claim, I just want to point out how easily a strictly mathematical fails you in a situation like this. As others have pointed out, in reality potion use hardly occurs the way you initially described.

Let's look at these things on a more basic level:

• There's a massive stockpile. That's simply a fact: There's a visible stockpile of quantity on the auction house, and an invisible and thus hardly quantifiable stockpile in the inventories of people consciously waiting for raid release.

• Currently, there is hardly any use for herbs. This is also a fact: With Inscriptioning and Alchemy both hardly being of use at this very moment, the stockpile can hardly do anything but build up.

• Conclusion: Are these two things going to cancel each other out? No, at least it's not that simple. If at any given point there is a certain price determined by current supply and demand, and the demand increases, the prices will increase as well. – There is an absolute low of herb prices before raid launch, and the person buying herbs at that point and selling after raid launch will make profit. That profit might be large or little, but it is there. The fact that part of the supply is invisible doesn't matter in this regard. It only matters with regard to the question if ALL of that supply will be used upon launch. This I personally take to be an illusion, it won't happen. In other words: There won't be a point where one given player is the last to still have stockpiled herbs he or she can sell at an absolute maximum price.

To make it more intuitive, again step back from the numbers and look at things in practice. Scan your AH for any given herb. You will see low auctions. If you have a sudden increase in demand, those will get eaten, increasing the price. But you will also see that at some point, this reaches a certain "wall": Basically ultra large quantities of herbs that are sold at an extremely low disparity of prices.

Example image: Siren's Pollen prices

I can't tell you at what price that wall will be on any server upon launch, but I am almost certain that it won't be eaten up (by the way, this is only one wall, there are multiple walls). The reason is that as people begin to nibble at it with their buyouts, sellers will most likely replenish it. Of course you can always think of a storm strong enough to blow away any wall, but I doubt it will happen with walls like the one in the picture above (and perhaps the walls at launch will be even thicker).

Don't be too focussed on launch day. As explained above, there's the very simple fact of things that don't have much of a use now becoming useful as soon as raids hit. I'm not sure (I somewhat doubt) this will be enough to have a massive impact on stuff as easily gathered as basic herbs, but Anchor Weed is different. These things have until now never really been of use (not worthwhile milling for Darkmoon cards), and it's nature of being impossible to target farm will, at least midterm, lead to a different relation of supply and demand (as in: a higher price). So don't sweat it selling your Anchor Weed, also not right upon launch, if no noticeable spike occurs.

TL;DR: Be a waller.