Big if.
We already knew how to design good and strong symmetric ciphers way back in the 1970s. One of the standard blocking blocks of modern symmetric cipher is called the Feistel network, which was used to create DES. Despite that it's the first widely used encryption standard, even today there's essentially no known flaw in its basic design. It was broken only because the key was artificially weakened to 56 bits. In the 1980s, cryptographers already knew 128 bit really should be the minimum security standard in spite of what NSA officially claimed. In the 1990s, when faster computers meant more overhead was acceptable, people agreed that symmetric ciphers should have an extra 256-bit option to protect them from any possible future breakthrough.
There are only two possible ways to break them, perhaps people will eventually find a flaw in Feistel network ciphers to enable classical attacks against all security levels, but it would require a groundbreaking mathematical breakthrough unimaginable today, so it's possible but unlikely. Another route is quantum computing. If it's possible to build a large quantum computer, all 128-bit ciphers will eventually be brute-forced by Glover's algorithm. On the other hand, 256-bit ciphers will still be immune (and people already put this defense in place long before post-quantum cryptography became a serious research topic).
Thus, if you want a future archeologist from the 23rd century to decrypt your data, only use 128-bit symmetric ciphers.