So, that's NOT the sunk cost fallacy. If you are still getting benefit from the cost, it's not sunk*. This is just not wanting to invest time into a new system while you're still productive in the current system. A sunk cost would be like, "Well, I spent $100 on this steak, so I'm going to finish it, even though the next bite may make me vomit." In that case, you paid for the steak, it cannot be returned half eaten,…
The fallacy is in persisting with it when other tools may be better, choosing to persist because of the time invested. Continuing with emacs because of that time investment even when there are better tools (for some part of the work at least) is an example of the sunk cost fallacy. Now, that doesn't mean that the tradeoffs can't be considered where someone determines that emacs is better to continue with for them in their particular situation (regardless of the sunk cost of time spent on it).