There are tons of reasons for people to switch back from p-servers to classic when it comes out (if the version suits them obviously): it's official, it feels way more authentic, you are more likely to meet more people but also friends that could come back for a while, there should be a way bigger playerbase overall, with everyone starting again at the same time, etc. And, let's imagine, if a small p-server sees its playerbase divided by two, then the rest will follow.
Now of course it might have less impact on other p-servers with a completely different version (let's say 4.6 hum), but still.
And this is also the only interesting way for GF to actually take back the playerbase that moved to those P-servers, because we perfectly know that they cannot kill every one of those servers with just a snap.
Now about the impact on Live servers. I strongly believe there is no such thing as "new players" anymore, so classic or not, the playerbase will eventually become smaller and smaller anyway, untill it just dies, it's unavoidable. Especially knowing that 7.9 (the only future version we know of) has only started to point its nose in Korea, with potentially a recycled Pangea of some sort, so NC is even slower and even more lazy "working" on Live content, so much that we might never get an 8.0.
I guess there are still people enjoying Live, but most of them must be already on the train with gear and content and P2W and etc. Everyone that is somewhat late in terms of progression will probably quit and try out classic, because there is no middle ground at all, you're either fully geared or you get destroyed without standing a chance.
If the trend is the same as it is in Korea, and we have reasons to believe it is, Classic will work, no doubt, but for how long? That we can't tell. And there are lots of key questions Gameforge needs to answer: like will they chose language based servers (which I think would be a major mistake, but it's only my opinion and I can totally understand people not willing or not able to speak with other communities that don't speak the same language).
Anyway, we know it's in the pipes, and it will probably happen, unless Classic Korea just fails and is deserted after a few weeks, which is still to be considered.