That's a really tricky thought exercise. Legal trusts in most countries have time limits. Businesses come and go all the time. I think you are describing the same dilemma that companies offering freezing/preservation of your body or head upon death/near-death are dealing with. That is probably the business model I would study to find out what countries have the legal structure to support their requirements and thus your requirements. Even then they and their clients are accepting some risk. I suspect you will have to invest some capitol in this if you don't want your future generations to carry on this project on your behalf.

If your time requirements were shorter I would suggest a legal trust and set requirements for trustees to ensure the domain and site are preserved. I would also add instructions in the site itself to have family members preserve the site or even create new domains or methods of presentation. Each generation of family member could then create their own trust and repeat the process through inheritance. Your lineage could essentially leap-frog the system and compensate for businesses going bankrupt or technologies changing assuming they value the site and wish to add to it. I think your future generations would appreciate the ability to update the site. "Keys change, technologies are updated..." -- The Davinci Code