The dictionary definition of consumerism is: https://www.merriam-webster.com/dictionary/consumerism
1: the theory that an increasing consumption of goods is economically desirable
also : a preoccupation with and an inclination toward the buying of consumer goods
2 : the promotion of the consumer's interests
This is also definition 2.1 from wikipedia (https://en.wikipedia.org/wiki/Consumerism):
Consumerism is the selfish and frivolous collecting of products, or economic materialism. In this sense consumerism is negative and in opposition to positive lifestyles of anti-consumerism and simple living.[5]
Previously, from context, I believe it's quite clear that we're talking about definition 1 b (merriam webster) and 2.1 (wikipedia). The original post talks about how consumption is good even if frivolous, according to the OP; I believe this makes that quite clear. This is why the definitional issue of consumerism isn't quite relevant, and the definitional issue that is relevant is regarding what's frivolous. I see this a lot in internet discussion, where discussion revolves around a concept that is encapsulated by a word with multiple meanings, and a different-but-related meaning of the word keeps being brought up. It muddies the conversation. The discussion is about the concept, not the word; words are but the medium.
Regarding your more on-point criticism, I generally agree. I think the key, so to speak, is two-fold:
“Consumerism” came up in my recent interview with Elle Griffin of The Post. Here’s what I had to say (off the cuff):
Can you steelman “consumerism” for me?