The Google Quantum AI whitepaper [1] established that the secp256k1 elliptic curve discrete logarithm problem (ECDLP-256) the signature primitive securing the majority of public blockchain networks can be broken with fewer than 500,000 physical qubits in approximately nine minutes on a fast-clock cryptographically relevant quantum computer (CRQC), a roughly 20-fold reduction from prior estimates. That analysis addresses cryptocurrency nodes and does not model the structurally distinct risk profile of IoT/WSN nodes participating in blockchain networks. This paper closes that gap. We demonstrate that gateway-mediated transaction submission, combined with the duty-cycle wakeup latency endemic to IEEE 802.15.4-class sensor nodes, can extend the on-spend attack window beyond Bitcoin’s ten-minute block time making constrained WSN nodes more vulnerable to on-spend attacks than standard cryptocurrency clients for certain deployment configurations. We introduce a formal HNDL Risk Score (HRS) that jointly quantifies harvest-now/decrypt-later exposure, on-spend vulnerability, and migration feasibility as a function of node hardware class and deployment lifespan. We benchmark all four NIST-standardised post-quantum cryptography algorithms (ML-KEM-512, ML-DSA-44, FN-DSA/FALCON-512, SLH-DSA-128s) on representative WSN hardware (Raspberry Pi Pico RP2040 and ESP32) and identify FN-DSA/FALCON-512 as the superior candidate for WSN-blockchain transaction authentication, offering 666-byte signatures at 71.6 ms mean signing time on Cortex-M0+ hardware. A concrete, tiered migration roadmap is proposed for healthcare WSN, industrial IIoT, and smart city sensor deployments.