The methodology of that requires the major number to change when backward compatibility is broken. That's the case with the new release. Not one style will work "as is", practically no Mods will function. Even the language file contents and structure are completely different.
If phpBB decides to continue with a 2.x line, Alpha will probably choose that one because it wont have all the bells and whistles of the 3.x line so it wont be a major resource hog on the servers (if that's still a problem).
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Psht, that's already true.
Megatron...
I still function!