Do you think that the forums will become as active as they were pre-launch and during launch again?

Well every content update will have a "Spike" in activity. That is a common factor for almost all games.

But if you are considering constant player base then no.
A constant user base is based on 2 major factors.
1. Player Saturation of the Product and
2. Market Saturation

The player saturation of the product can be looked at how much time a user spends on the game during beta. After a variable period of time, even if new content is added he or she will not go back to the game for a prolonged time. Maybe a week or so, but then drop it again.

Because of this many people have purposefully stopped playing the game, waiting for release. So they do not get saturated by the game.

Market Saturation of the product is the number of users who are "willing" to buy starbound. An how much of those have already bought it. If you have 1.5 million people who are interested in the Starbound in total of present past and future. And 1 million have already bought it. That leaves a open market of only 500,000 left. Out of those 500,000 only a fraction would buy it, for what ever reason. There is also a chance that upon release starbound will get new found fame. It really matters a lot on social dynamics and what people want. For the past few years quick casual crafting style games were the "in thing". Whether that will still be the case in another year is anyone's guess. But CF is betting on it.

The final determining factor of how popular SB is going to be on release is what direction they will finally take with game design. Bad locked progression mechanics can seal the deal for the game, so it hits market saturation much earlier than expected. So we will have to wait and see how CF evolves the game - and by stage 2 we will have a much more accurate picture of the future of SB.