“A time machine will and can never be invented. I know this because no one from the future has ever come back here.”
Or they might not find an advantage in revealing their presence. And specifically, why would they seek out Az Joe to convince you personally that time travel works?
But that assumes that these "future" people would ALWAYS feel that way about revealing themselves, each and every one of them, eternally for many, MANY millenia, eternally, forever and ever nd ever. No one in this future, with time machines in existence, would EVER reveal themselves to those in the past? EVER?!
Seems extremely unlikely. My contention is the more likely one. There will never be a time machine because we would have already been visited by people from the future...and we have NOT!