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.
Actually, there's another way to do it. It's what I am doing, I just didn't realise it until now. :)
Reserve a room in your house for your own computer history museum. Use old systems, running old software, doing old tasks that you learned years ago. Never update, never upgrade. Each time you need something new and it doesn't work anymore on the systems that you have, buy another new system and add it to your collection. Keep using it for that task and newer, while continuing to use the old systems for the old tasks. Virtual machines work too.
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.