There was plenty of 'room' within each of the alignment planes, though. 'Layers' for instance (the Abyss had 666, it's not like many of them were defined), and each layer could be infinite, as well, with plenty of room for whatever you needed to add. Then there's the concepts of deities having different aspects based on the beliefs of their worshipers, so different versions of a god of death could all be 'true,' even if they were all the same alignment (ie just perceiving the same god differently). Similarly, you could plop any number of new Domains in the Astral Sea, if you wanted.
Similarly, the inner (elemental) planes got increasingly sub-divided.