It's no replacement for envy because it doesn't fit.

that doesn't stop it being a replacement, although you could make a good case for it being a poor replacement because of that

But by that logic anything could be a replacement of sorts, even if it's worse than nothing.

yes. well done, you're starting to get a rational grasp of the concepts.

But can something that is actually worse than nothing be considered a replacement at all? Of course, this whole argument depends on the definition of replacement.

the answer to your question is 'yes' it does depend on the definition of 'replacement,' but the definition of 'replacement' does not depend on whatever we think it is, it is something which replaces something, that is all, it doesn't have to be any good.

True, anything can be considered anything. Definitions are not absolute however, they do not for example take connotation into account, which has a huge bearing on the meaning of a word.

they have NO bearing on the meaning of a word, that is a childish mistake, even though the word 'replacement' as used in the wild almost always refers to a replacement which is suitable is some way to substitute for what it's replacing.

But a word's meaning is derived from its usage. Connotation, over time, becomes part of the meaning of a word. Fantastic for example. It originally meant "relating to fantasy", however it has evolved to mean "brilliant" due to an extended period of time where it was only used and interpreted in a positive manner.

fantastic has not evolved to mean 'brilliant,' 'brilliant' means, like, bright and shiny

Ok, fantastic has evolved to mean extremely good.

yes - fantastic now means extremely good because the definition has changed, in contrast to replacement which doesn't mean a suitable replacement because the meaning hasn't changed.

But fantastic is an example which proves that connotations can have bearing upon the meaning of a word. The current use of language suggests that the meaning of replacement is evolving to imply suitability, like fantastic has changed to mean extremely good.

the definition of a word and what current usage suggests the future definition of a word might be are hardly the same thing

True, however the current usage includes suitability and is generally accepted to do so. Language is primarily about communication, so to treat a word as to have the meaning that the majority accept it to have is not inherently wrong, even if it disagrees with the definition slightly.