Absence of evidence isn't evidence of absence. Credible testimony of eye witnesses is as good proof ad heing caught with dope in your system. People get found guily of crimes all the time without any physical evidence.
On the flip side 500 tests and all of them a pass is statistically improbable. All the eyewitnesses are drug cheats themselves, what deal have they cut with USADA to testify?
On the flip flip side, if as one famous drug cheating coach said: 60% of olympic athletes are drug cheats so only a small fraction of drug cheats get caught. Lesson, if you're smart then you won't get caught no matter how many times you are tested. Armstong, and his crew, are probably smart enough to stay ahead of the testing ball. e.g. Armstrong may have been using EPO before WADA developed a test for EPO, then blood doping before they started testing for that, then... there's always a new performance enhancing drug on the market and until WADA develops a test for it you can use it with impunity. If the drug cheats who are testifying against Armstrong are not gettig any deal from USADA then their credibility can't be questioned too much, although jealousy is a very strong and evil motive for wanting to bring someone down, it is figuratively an all consuming fire than can burn away all traces of decency.
So, lack of physical evidence is meaningless if the eyewitnesses are credible. So it all comes down to that.
Also of note is WADA supports USADA's course of action, and the UCI have a vested interest in seeing their greatest poster child not fall from grace. So no one has totally clean hands in this.
Armstong should listen to Naruto, if he's innocent and "never give up, no matter what!"
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix