I could only find this blog that suggests that complacence is ” is a calm satisfaction with oneself ” whereas ” complacency means a self-satisfaction but coupled with a lack of awareness of what is happening around it. For

what reason is there a difference between the two? What is the grammar of different endings for a word?

I just remembered that stagnation is probably the preferred word, so perhaps ignore the stagnancy vs stagnance part (stagnancy seems to be entirely incorrect anyway).