This would ease the migration between extensions: you could install the new extension in your system and load the emulation library so that code depending on ext-mongo could work (allowing you to migrate things progressively), without having to install both PHP extensions (which would not be possible at all if the conclusion of PHPC-174 is to reuse ext-mongo for phongo btw), and without falling back to mongofill (for performance reasons).

The old API would then be considered as an "high-level" API on top of the new driver (using it would be discouraged though)