"A few days" is anywhere from three to six, so it isn't really as specific as one might think.
You can't really have the idea that "a few days" is somehow less specific than "bags" as a measurement. If 'a few' is unacceptably ambiguous, then so is 'bags'. Is four bags four thousand, four mining bags, four gem bags, four bags of goblin teeth, four old women, or what? This ambiguity is even funnier when people make these statements while standing to three or four grain sacks of a different size.
Language evolves and adapts, and cultures develop their own slang. This is no different.
Language does evolve and adapt. Cultures do develop their own slang. The problem here is that the concept of "1 RL day" does not exist in character. Period. 1 RL day is an ooc reality. 3 ingame days is an In character reality. Our characters are not supposed to be aware of OOC things because they're out of character, and therefore, the in character language should not morph around those factors.
Trine is simply put, a way of acknowledging an OOC conversion factor ingame. It's commonplace because it takes less work than pausing, thinking about what you're saying, and do the math to go, "Well, I'll be on in x hours, divide by eight...oh, that's about three and a half days." It's frustrating to some people because in English it's bizarre to organize days in groups of three with its own special word. The only reason that the number three is used in this case is because of the OOC conversion factor.
Look at it this way. Go to your boss and tell him you'd like to take a trine off. When you're done explaining what a trine is, you'll be asked why you didn't just say "I want to take three days off." There is no reason to organize days in groups of threes like this.
Language does not evolve around factors that don't exist. The factor of 1RL = 3IG does not exist in character.