Are non-supersymmetric GUTs ruled out due to lack of precise gauge coupling unification? Does there exist any good proposal on how the gauge coupling unification can be fixed in non-supersymmetric GUTs? If not, can we assert that non-supersymmetric GUTs have been experimentally ruled out? Of course, I'm referring to the category of GUTs not already ruled out by proton decay rate.
P.S. Somehow people take this issue as evidence for supersymmetry, but not as a death blow to non-supersymmetric GUTs. Why is it the case? Is there a simple fix?
 A: There is the possibility to have particles at an intermediate scale which correct this dilemma, but this is not very appealing since the new scale is essentially a free parameter which takes away the cool GUT prediction for the strong coupling. Another alternative to supersymmetry is having other additional light particles at the TeV scale in the correct amount (for example four weak scalar doublets or something like that has been discussed before) which can very much improve the precision of unification in nonsupersymmetric models, and since there is more or less only a discrete parameter (the number of light new fields) it kind of is predictive and falsifiable, since those fields should in principle be observable.
I suppose the main reason why many people are scared away from such ideas is that one explicitely introduces new physics at the gut scale without solving the higgs hierarchy  problem. That means that at the GUT scale, you really do have one parameter in your theory that has to be tuned to many digits precision in order to get a small electroweak scale.
Hope this helps
Cheers
