That's equivalent to saying that a scientific claim doesn't have to be falsifiable. That's not true. A claim has to be falsifiable for a reason.
> If it has not been proven, then it does not exist.
So the Higgs Boson didn't exist six months ago? It wasn't proven, because there wasn't enough evidence for a statistically relevant result until March 2013. So does that mean that prior to March, you would have concluded that the Higgs Boson doesn't exist?
If I had to sum up my problem with your reasoning in one sentence, it would be "belief is scalar, not boolean." Evidence should adjust your belief state somewhere between 0 and 1, but it should almost never be exactly 0 or 1.
> Just as until unicorns are proven to exist, it is correct to state that unicorns do not exist.
That's a variant of the "existence of god" problem. It's a non-falsifiable hypothesis. That doesn't fall within the realm of science, because it's not subject to empirical evidence.
> My opinion is based on the evidence.
I still haven't seen the evidence to support your conclusion.
TDD's claimed benefits are falsifiable. Much like the Higgs Boson prior to March 2013, they have been neither proven nor falsified. But that doesn't mean we can't form a tentative belief based on the weight of the current evidence available.
The weight of the evidence I've seen leans toward indicating some real benefits beyond normal unit testing. It's inconclusive because the studies lack statistical power and procedural consistency, but that doesn't mean we should automatically jump to a hardline decision in either direction (positive or negative). (Again, belief isn't boolean.)