I think a central problem around "completeness" as a term is that it's not synonymous with "playable", as some people are supposing. "Completeness" and complexity are opposed to each other; the more rules you add to a game, the more incomplete it becomes.
Like, this example:
1) There is a player and a DM.
2) The player makes a fictional persona, i.e. a character. The DM explains what is happening to the character, a scene.
3) The player explains what the character is going to attempt, and the result if they succeed. The players then flips a coin.
4) If heads, the player's character succeeds at their attempt and the intended result in realized. The DM then presents a new scene.
5) If tails, the player's character's attempt fails and the DM narrates the consequence of the failure and then presents a new scene.
That is a complete game. Any character or setting is playable, and any possible action has a system-defined method of resolution. But, I very much doubt anyone wants to play that complete game over the relative incompleteness of 5e; the reasons for that preference are probably determined by your motivations for gaming.