Well, anyone could yes. That's why if you want messages you're sending today to remain secret for longer than a decade, you use a cipher that isn't fully broken

Well, anyone could yes. That's why if you want messages you're sending today to remain secret for longer than a decade, you use a cipher that isn't fully broken by quantum computers like AES. It's really just the key distribution methods that will be broken by quantum computers, they only get a sqrt(n) speedup against symmetric-key cryptography: double the key size and you're golden.