To add to Stompa's point.
If servers merged, one of 2 things would happen with dynamis:
Prices would plummet as everyone can do dynamis and the server gets flooded with coins. Or skyrocket as dynamis zones get filled and coin income slows from competition.

Plus adoulin would just be overflowing, which is where everyone is right now, would be as bad as jueno used to be in 2005, stuffed full of people.
Remember, More is not always better. some things it might fix, but others are going to get messed up, and mergers involve a lot of hard feelings from players due to name loss (I lost Amanda which I had since 2003, when Alexander merged into Shiva), shells, and friends. Loss of friends means loss of funds for SE to support FFXI, because both the person who left AND their friend might go, and others will do the same, it's a potentially large snowball effect.

I wish we could see signs of life in older zones. Walking through most zones I go leveling in off pages is like walking through town after a plague. But just adding more people isn't going to solve that. The key to both problems is adding content and zones, people want to do, and adding enough of it.

Most people are too impatient to level the old way when they can burn to cap in a few days, so getting them back into older zones requires things like Unity does, or magian trials attempted. Getting players to come back is going to require a lot of work, like a new good expansion pack loaded with at least as much as adoulin had, if not more, and a story to rival CoP. (And empy armour reforge!! Seriously!)