Potential Service Life of ChaCha20?
There's simply no way to know. I find it very unlikely that AES will be vulnerable to a ciphertext-only attack in the next 20 years (remember, it has been around for over 20 years already and attacks haven't gotten very far). There's no reason to believe the same won't be true for ChaCha20. If you use a good cipher with 256-bit keys (to avoid potential speedups from quantum computers running Grover's algorithm which halves the effective key size of a symmetric cipher), you should be fine.
If you really, really want, you could use something like VeraCrypt which provides you with the option to use three ciphers in a row: AES, Serpent, and Twofish, all of which are extremely secure.
We can say with some level of confidence that it will remain secure for your purposes.