Right now the only two points of contention are the xp rates and the impact of cheap ess on runecrafting. When they nerf nmz, surely less people will do it, or (if they remove runes and you have to bring your own) people will be less willing to sell their ess off for nothing. But for the time being pure ess could go even lower because supply just continues to exceed demand day in and day out. Merchanters hold such a high volume of pure ess right now. They basically absorbed the massive supply of ess.
Given that a nmz nerf is at least a couple of weeks away and there is currently no speculation affecting the market, you are better off just cashing out your points asap. As soon as they repoll it and it's clear that it will pass that week leading up to it will see prices of all materials rise as nmz players hoard their points, as runecrafters stock up on ess, and as merchants try to get a free lunch.
If you can afford to hold points, then it's up to you. The only for sure investment advice I could give right now is going out and getting things like potato cactus and dragon dust with gp. You can easily them for 70% of what people are selling on zybez, and in the case of cactus not many people are even spending points on them so any nerf whatsoever would cause them to rise.
Keep in mind a lot of zybez offers for the herblore stuff is people just advertising the item and then if you say you wanna buy then they go and convert the points.