Everything is increasing in price and what we pay to get access to 100m tracks of hi res music is still incredibly cheap. For comparison, Netflix (without ads) is roughly the same costs and you only get to choose from a subset of their catalogue at any point in time.
While I don’t welcome the increase, I’d miss voice control of Amazon Music in my car, as would the family miss it on their various voice assistants. Plus a family plan is still very cheap per user per month