What do you mean by "locked down computer." Maybe something like ChromiumOS?
Might be a tough sell for the volunteer open source community ("linux & friends") to work on such an alternative "locked down" computing experience. Free and open source software is usually more focused on unlocking use cases, not locking them up.
That all said, I basically consider macOS to be a locked down computing experience. So that's my solution for older people.
It's not a perfect solution but the Apple closed ecosystem is better designed for the limited use cases of the elderly. Rely on iCloud and built-in Apple approaches to data security as much as possible.
For example, an iMac and an iPhone can get all "adulting" use cases done, including typing/receiving emails, printing documents, online banking, government services, and so on. Apple Passwords plus Face ID helps to simplify password-based security. My biggest issue is getting TOTP-based two-factor adopted. Apple Passwords supports this but I usually have to do remote tech support to get it set up initially. It's also annoying that right now, the current generation of iMacs don't support FaceID, because that would simplify authentication across the two primary platforms (desktop/mobile).
I would never use this setup myself since I like to run F/OSS everywhere as much as possible. But I am realistic about tech expectations for the elderly who just want to live their life with minimal investment in learning about data/software security.
But you're right, along with other commenters, that it's dangerous for society to rely on a monopolist technocorporate overlord (or a pair of overlords forming a de facto duopoly) for the basic administrative tasks of adult living and lawful citizenship.