Nasal weakening

Languages seem not to like nasals. They're usually forced to assimilate in place of articulation to a following stop, sometimes even a fricative, and often they're dropped entirely (which leads to nasalized vowels as separate phonemes, e.g., French bõ or Brazilian Portuguese sãõ). In some languages they are dropped entirely (an areal feature of a group of 5 languages from the Chimakuan, Wakashan, and Salishan families of the northwest coast of North America). Even in languages with strong nasal functional load, nasal consonants are often weakened in some way. In American English the flapping rule applies not only to /t/ (bʌɾəɹ for "butter"), but also to /n/ (ɛɾ͂i for "any"). In Portuguese (at least Brazilian Portuguese), the palatal nasal (represented orthographically by the digraph "nh") is often reduced to a nasalized vowel + palatal glide, so that a word like "minhas" comes out as mĩjas.

If we think of this in terms of Optimality Theory (which, honestly, is what I'm doing always, even in non-linguistic things like traffic patterns and evolutionary biology), we can talk about two constraints: Exp(ressivity) (a sort of catch-all constraint I've been using half-seriously, with a definition something like "language should be able to express a speaker's thought accurately") and *Obs(truction) (something like "sounds should obstruct the vocal tract as little as possible"). The desire for expressivity is clearly what drives the robust distinctions between consonants after thousands of years. Though some will dispute it, there is an undeniable tendency for languages to simplify, espeically phonologically. This is how "want to" becomes "wanna," how "going to" becomes "gonna," and further how "I'm going to" becomes "I'ma." What holds back this march of simplicity is the Exp constraint. Simplify things too much, and people won't be able to express themselves properly, at least not without long strings of the same consonants/vowels.

So what languages seem to do is reduce nasal whenever possible. The reason this doesn't usually turn into nasal deletion or any other radical change is the necessity to use language as an expressive tool.