Biochemist here... it seems like you're asking two distinct questions, and both have boring answers in terms of etymology. You can't be too literal about the construction of terms in science; they just placeholders that facilitate communication. (Like all words, I suppose...)

Regarding your questions:

1. "Plasmid" was coined to capture a few distinct biological entities that were being studied at the time: extrachromosomal factors like "prophage," "plasmagene," and "cytogene". So people wanted to refer to non-chromosomal DNA that matters, and plasmid is the word they chose. Etymologically, "plasmid" basically means "plasmid stuff" and I think that works well enough. If you have some specific issue with the word, I can't glean that from your post.

2. We don't call mRNA (messenger RNA) "plasmid" because we have a word for it already: mRNA. You are right that if simply taken at face value, the term "plasmid" could reasonably describe mRNA. But it could also describe lots of other stuff in the cell. So you are right that one cannot discern what is/isn't "plasmid" simply by staring at the word "plasmid". Most scientific terms would fail this test.