What, in your opinion, would be a good hw+sw platform to build an appliance (offline, not cloud-connected in any way) that could conceivably keep working for another ~20 years?

For context, I used to live near a drawbridge, and its motion was controlled for 30+ years by a DOS-based computer (https://www.seattletimes.com/seattle-news/seattles-university-bridge-undergoing-a-reboot).

I'm sure there are very knowledgeable folks here, who might have built such long-lived systems, and I'd love to hear about platforms that are (relatively) future-proof.

Thanks!