Everyone tends to forget that it HAS been reconsidered. Remember that the original stage plans called for the following:
Greenflower, Techno Hill, Deep Sea, Mine Maze, Rocky Mountain, Red Volcano, Dark City, Doomship, Eggrock
The entire game was "completed" around 2000 with these level concepts, but as said before, the scope of the game expanded significantly and those old, crappy designs were scrapped around the creation of Demo 1 in favor of far more involved, interesting levels. While they're considered dated nowadays, GFZ2 and THZ1 are the first stages that were completed in this new model. Remember that this was the old THZ1, which was essentially GFZ4. As time went on we created the "demo-exclusive" Castle Eggman, which was actually always intended to be a throwaway concept invented to provide a reward for collecting all the emeralds in Demo 4, believe it or not.
When THZ2/3 were created and CEZ was put into the normal level order for Final Demo, the plan was still to use the original level concepts above. During Final Demo's eternal refinement cycle, this is where the lack of ideas for Mine Maze and Rocky Mountain and the rethinking occurred. Mine Maze and Rocky Mountain's concepts were merged into a new stage called Arid Canyon, unifying the concept through adding a desert concept. Castle Eggman's theme then ceased to be demo-exclusive and took the extra zone slot. Doomship was renamed as well to avoid the DSZ abbreviation being used twice. Hence, now the plan is:
Greenflower, Techno Hill, Deep Sea, Castle Eggman, Arid Canyon, Red Volcano, Dark City, Grand Eggship, Eggrock
Obviously, we have a bunch of secret stage plans that include a bunch of other themes, but because of the overactive mind of a 13 year old making way too many to be remotely practical, all the excess terrible ideas have been dropped and only the neat ideas have been kept along with a few new ideas. An ice stage has been here the entire time, along with a bunch of others like Azure Temple.
I guess what I'm trying to say in too many words is that the TUDD is a guideline, not a hard rule, and we can and do change it when necessary. Because of all the changes to the game over the years, the TUDD would be completely useless and 15 years out of date if we didn't do so.