The grand wishes of the Three Realms are akin to asking for a loan from the Heavenly Dao. Once they're made and approved by the Heavenly Dao, a considerable amount of Merit is obtained.
Therefore, the grand wishes of the Three Realms must be fulfilled once made. The Heavenly Dao also continually urges it forward.
With the Heavenly Dao watching, who dares to defy it?
As for the Hongmeng Grand Wish of the Great Wish Technique, as its name implies, it's simply a wish — even if it's an enormous one.
Wishes, as we understand, abound in a person's life. But if they can be fulfilled, that's another matter.
As long as you don't make grand wishes, asking the Heavenly Dao for Merit, there's no complication. You can make any grand wish you like.
The Hongmeng Grand Wish of the Great Wish Technique is just like bragging. The bigger the boast, the more powerful it becomes.