The limits of modifying one's own reality isn't limited to just popping back into existence unharmed. Forcing his lower half to regenerate to be his new reality is totally plausible.
When Danzo used Izanagi, the physical reality of him dying was erased, the spiritual was not. (The memories.) Izanagi can only alter what has happen, not create a new reality from scratch. Meaning that if Madara wishes to create a complete body out of the legs that are left he would need to alter the reality of of being cut in half to begin with. (If he does that then either his legs will be gone as he would have been made into Kaguya regardless) or it won't work as it would affect the reality of Kaguya and Black Zetsu as well. What you are stating is that with Izanagi a user can create a new reality of their liking, that is not what Izanagi does.
your bolding and underlining is pretentious and gay as hell