I already somewhat answered that somewhere else.

The choice is between stretching yourself thinner but still giving customers of all modules the opportunity to utilize the Georgia map or leaving Georgia be and patching the new modules just into 2.0+.

What you need to consider is the following: The reason for the multiple versions we have right now is the interface to the terrain, that is, how the graphics engine gets the data it needs for rendering terrain features (textures, elevation mesh, etc.).

If you're developing something that works with both the old and new terrain interfaces or just doesn't depend on the terrain interface at all, you can basically just throw it into the 1.5 branch and it should work, so it's not too bad. The stuff where this doesn't work, they are probably going to hold back until they can ditch 1.5. The way i see it, what they are doing right now mostly creates overhead in testing, not so much in development, but i don't know the source code, so take that with a grain of salt.

What i can say with absolute certainty ED is not doing is developing three completely different versions. They develop one version (2.5) and invest some additional work into backporting some of that development work into 1.5.