Ask HN: What is the best way to future-proof a website build?

It seems to me like most websites will get completely rebuilt on average every 5 years or so and if they haven’t been rewritten, they have become a ball of mud of outdated CSS and tightly coupled CMS. Most websites don’t get replaced piece by piece, they get driven into the ground and aren’t replaced until they are painfully out of date.

Imagine that you have been asked to start developing a website that will definitely still be in operation in 20 years. A non-trivial site that can evolve gracefully over time to incorporate new technologies and new features, is not prohibitively expensive to maintain, and the future maintainers don’t wish you a painful death.

Is it possible to do that right now or is it foolish to even try to predict the future? Are there any strategies or techniques that would help with this task? Static HTML generation? Microservices?

Is web development still too immature an area to expect this sort of long term stability? Will it always be cheaper to just do the big rewrite every few years than to do slow and steady maintenance?