There is no proselytizing going on here my friend. The op asked how to prove to an atheist that God exists. My reply was that is what prophecy is for.
*ahem* Please re-read the OP and note that I make no requests of proof from anyone. What I offered was what
I would consider conclusive proof, predicated upon the scenario I outlined within the OP.
It's really not a complicated or overwrought proposition.
Believers that wish to pray for a miracle and beseech their god(s) for some display of supernatural (defies or is beyond any natural explanation) cause and effect, that I believe would instantly convert many millions, if not billions of heretical unbelievers, or at very least...me
The only inquiry posed in the OP was "What's wrong with that?"
No piety.
No evangelism.
Not trying to convert.
Just an answer to the OP.
When your "answer" is little more than a list of prophecies and yes, cautionary predictions and the like, it is both a testimony of piety (if you didn't believe, you wouldn't recite prophecy as "truth"), which is actually what Evangelism" defines ("the spreading of the Biblical gospel by public preaching or personal witness".)
Whether you seek converts, well, perhaps you do not. But it is worth noting that all Abrahamic religions deem the teachings of Jesus as being either a rabbi, prophet, or son of God himself...and Christians are charged with Matthew 28:16-20, to indeed do as they are commanded.
If your faith lets you off the hook, that's fine with me
Don't look if you do not want to see.
I've been looking and keeping notes from post adolescence, nearly 40 years now.
So, I again offer you the OP and the lone question it asks...
"What's wrong with that?"