ot that I'm denying the existence of a fifth... but what is it? length, width, depth, time... what? even time is still up in the air.
Length, width, depth, time,
charge (yeah). Seriously, why do we need a specific name? It's just the fifth dimension.
What about the other forces, el.-weak + QCD? So far no derivation from theories with extra dimensions.
First of all, you can make
any gauge theory by adding a manifold of extra dimensions homeomorphic to gauge group. It's not very elegant, but it works.
In a more civilized way: EW is not that hard to embed into KK. Fifth dimension might be actually hypercharge or something. Even dilaton can provide Higgs mechanism, I read.
As for QCD - indeed I did not hear of any nice theory. However, I would be satisfied if for the next century QCD was just a field theory on a KK background.
Not very unlike Standard Model. It's also just EW+QCD forcefully glued up.
Another question: the topology of the fifth dimension is fixed. Is this a stable solution taking into account the singularity theorems of Hawking & Penrose?
I don't know.
Any theory has some weaknesses. KK is not some paradox-free magical book. It just stands for its simplicity and elegance. One simple assumption leads to a cascade of theses so that Standard Model emerges naturally.
Don't rush. Kaluza-Klein will show it's glory some day.