Interesting suggestion. It is possible for mā + icchāmi to be micchāmi in Ardhamāgadhī (or pseudo-Ardhamāgadhī) because lopa-sandhi is common in such contexts (see Pischel §158), but since the phrase occurs in the context of the pratikramaṇa, and in a few cases specifically refers to the evil of an already-committed transgression (jaṃ khaṇḍiyaṃ jaṃ virāhiyaṃ tassa micchā mi dukkaḍaṃ), it seems more natural to me to read it in the traditional way rather than as a resolution not to desire evil deeds in the future.