obviously, you don't understand the movie.
I can prove beyond a shadow of a doubt that you are wrong in your assertion.
Simply look at Cobb and his wife and all the statements made about 'limbo' (and also dont assume that everyone, especially Cobb is 100% truthful or correct in their assertions in the movie).
Limbo is more drugs than multiple level dream (although multiple level dream does contribute to getting lost in limbo).
i.e.
A single level dream, drugged and can't wake up. Let us say you die while dreaming. This should wake you up but due to drugs, you don't. However, you THINK you do. If the drugs/multi-level are long enough you can be stuck in dream space 'forever' (without knowing that you should wake up) to the point that your real body dies/decays, etc. Either way, the whole thing being a dream has several parts of the movie support it. To actually argue that it is a "true happy ending" with no limbo at all (whether from start or no escape) is actually the least likely case.
This also involves discussion and understanding of "dream space" in the sense that in every "dream experience" you have a primary dreamer (architect) and a primary subconscious (person who populates the world). Architect makes the world, subconscious populates it with the NPCs, etc. and that this distinction breaks down in limbo.
Corvuus
p.s. also notice that limbo is reached (according to the people) mainly from dying while in dream while drugged, not dependent on multiple levels since if ANY of the people died in any level of the multiple layered dream, they would have been in 'limbo'. Also note that limbo is the 'big bad boogie monster' to the dreamers since no one ever came back 'except' Cobb.