Not possible.

The most important thing you will need is to access digital banking and government services. These apps already requires you to have an updated and locked-down system.

If you disregard that requirement (let's say you'll replace a tablet periodically, just for banking and government services), the next important thing you will need is communication. That means an updated browser that can open the future generic wordpress-based website, discord, twitter or whatever platforms will be popular when you'll be old. These platforms already require an updated browser, and updated browsers require updated OS, which require updated hardware too.

You don't actually need a "completely stable computer system". What you need is a secretary.