As I get older (and watch my parents get much older) I see a desire for a completely stable computer system that continues to work exactly the same way for timescales of decades. Currently this is impossible in the Windows or Mac ecosystems -- the operating system will go out of support, and force an upgrade, and then things will work differently. And maybe the upgrade requires new hardware, and then there is no way to bring your old programs across and hard to bring all your data across and then everything is different. Maybe its possible in the linux ecosystem, though there sure are a lot of people saying you have to stay on the update treadmill or else you'll get eaten by the script kiddies and their internet worms.
Maybe there is an approach using something like BoxedWine, to have a stable user-facing system that can be 100% ported to new hardware or underlying OS?
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.
As I get older, I accept that nothing humans make can last without maintenance, and that for computer systems in our growth economy, that maintenance has to be equivalent to change.
It sucks, but the repeated aggravation of broken expectations is worse.
That doesn't mean it stops working, and I know there are plenty of communities providing unofficial support for old Windows and even DOS. The situation with Mac may be similar but smaller, just due to relative popularity.
While this is mostly true, with how much software is web based, and how web browser support is essentially only on supported OSes, OS support does matter.
This is less of an issue for DOS and Windows 95/98 because software in that era was offline-first.
The trick is to put an interface computer between the (user) computer and the internet.
The user computer remains unchanged through the decades. The interface computer is updated with all the security updates, and communicates requests back and forth between the user computer and the internet, ensuring format compliance. It also contains software to convert file formats from newer version to the user computer compatible versions (where possible). For example, converting Word document format to some old version.
