From let's say 1000 years from now, would it be feasible for someone to use brute force to find a public key with a balance?
No.
It is possible that some time in the next 1000 years some mathematical weaknesses may be discovered in come of the cryptographic algorithms used to calculate an address from a private key, and those weaknesses might make it possible to start with an address and calculate a working private key for that address.
However, it will not be possible to use brute force to find a private key for any truly randomly generated address.
I highly doubt it. Most addresses are only used once and the actual public key is not known until it is spent and thus emptied.
To find a public key from an address with value, you'd have to break both sha256 and ripemd160 - and that would only get you the public key. Then maybe a mathematical weakness could find you the private key.
I do not see that happening. The odds are much higher that we will all wake up tomorrow morning to find that the core and BU developers had a secret meating that resolved all their differences...