Has this ever happened before?
With a properly generated private key?
No.
If there becomes 100 trillion wallets in use, do you think finding a wallet with a balance will become common?
100 trillion is a VERY small number. The answer to your question is no.
Is the current private key secure enough to last 10,000+ years?
Assuming that no new weaknesses are discovered in the ECDSA, SHA256, and RIPEMD160 algorithms, the current private key is secure enough to last more than 14 billion years (note that the entire universe is probably less than 14 billion years old right now).
If weaknesses ARE discovered in any of those algorithms, then the algorithm can be replaced before weaknesses are discovered in any of the other two.
Is there a way to scale up the security of the generated private keys somehow if needed someday?
Yes, but it might require a hard fork.