Language change

 Here's something: language changes because everything changes. Musical styles, clothing, beliefs, food. There is a drift in everything. I'm sure Catholics don't believe the same things as 100 years ago. There is some stability in some things, but overall human culture is not a stable system, and requires huge effort and institutional power to resist change.  

There are immense time spans, and geographical spreads. 

Language has a large number of moving parts, and hence is especially vulnerable to entropy. Phonemes, lexical items. The larger the number of variables, the more unstable the system.  Morphemics and syntax are relatively stable, in contrast to the lexicon and phonetics. I'm thinking that Shakespeare's grammar is pretty much mine, leaving out the thou conjugations. If I fail to understand Shakespeare it will be matter of lexicon, not grammar.  

If a person know 20,000 words, that person will not know 20,000 of anything else. They won't know 20,000 kinds of spices. If they do, then they will know 40,000 words. The language is more or less co-extensive with the territory.  The size of the lexicon is elastic. It can expand with more words in a particular category. There have to be more nouns than verbs, I'm thinking.