A user-owned, decentralized “post-platform” internet is emerging, but looming quantum computers could strip it of privacy by cracking today’s encryption. Shor’s and Grover’s algorithms place RSA, ECC and even 128-bit symmetric keys on borrowed time, prompting a “harvest-now, decrypt-later” threat. NIST has already standardized lattice (Kyber, Dilithium) and hash-based (SPHINCS+) algorithms, and real projects; from Matrix to Nym and Fediverse pilots, they are weaving them into protocols. The safe migration path starts with hybrid handshakes, moves to full PQC once middleboxes and hardware catch up, and must finish before regulators’ 2035 deadline. Builders need to integrate open-source PQ libraries, audit implementations, and pressure vendors for quantum-ready HSMs, because if we wait, tomorrow’s quantum adversaries will turn today’s decentralized traffic into plain text.