A thing I've noticed with groups of posts chopped or moved to new threads is that they can lose part of the original context.
I saw the original query and discussion as twofold. There's the dish in general, it's originality along with its cooking method, its characteristics and ingredients and the experience and enjoyment of eating it, variations, and the question of substitution of an ingredient.
The thread becomes about a single ingredient which makes the topic much more narrow. Absolute authenticity of a regional dish is a topic. Another is about making a dish with most of the original attributes that is nonetheless in the same ball park and still authentic, but a variation.
I too think annatto is a extraordinary spice. After having bought a small package the first time I made Cochinita Pibil I was blown away and ended up buying a large jar of the stuff and use it in many dishes, many Mexican and Latin that take on a unique depth of flavour that is also described by its colour, its visual flavour.
I also don't think the annatto is the make or break of this dish and think you could make a pretty good Cochinita Pibil version without annatto. It would probably be a little closer to pork marinated and roasted in an adobo paste - ground mild chile, spices, garlic, vinegar etc, but the experience would be there, like a regional variation.