I'm sad. I'm a software guy, not much of a hardware expert, so please bear with me if I"m too pessimistic. However, I feel that if trends like this continue, it might be the end of enthusiast-level personal computing as we know it. No more being able to head down to the electronics store and purchase RAM, motherboards, processors, GPUs, storage, and other components. We're going to be limited to locked-down terminals…
- Hardware
We won't have any hardware without secure boot and we won't have the signing keys. Signed firmware is required for everything x86, everything Apple, everything mobile, probably everything ARM too. Rockchip ARM could still boot without signed firmware last time I checked a few years ago, but I'm not sure about the newer CPUs.
[ Short story: I have an Asus Tinkerboard S. It came with an unprovisioned emmc onboard. Just empty. A few years ago I made the mistake of trying the official OS from Asus. It automatically set up the CPU to boot from emmc first and provisioned the emmc with boot0, boot1 and rpmb. These partitions can't be written and can't be removed. Their creation is a one-way operation according to emmc standards. Now I have to keep the emmc masked because it won't boot otherwise. So beware of any devices with emmc. ]
You can, of course, use MCUs for general computing today. ESP32 is pretty powerful - it's probably 4 times faster than a 486, certainly more powerful than an i386 or a 68000 you suggested. The big problem here is memory, graphics and software. No MMU requires a completely new OS. Linux (uclinux) could boot at some point without a MMU, but it won't fit in 540KB memory. MCUs can access external memory (PSRAM), but via slow buses and it's paged. Also there are no hi-speed buses for graphics.
There is some hope coming from the chinese CPUs. AFAIK, they don't support secure boot at all. I'm planning on getting one as soon as their proprietary firmware/UEFI/ACPI and be replaced by uboot.
- Software
It's useless to make a i386 or 68000 today. There's nothing but old software for them. Not even linux has i386 support anymore. This is an even bigger problem than hardware. Much, much bigger. To have any hope of a minimally useful computing platform, we need a working browser on that platform. This is an absolute requirement. There's no way around this. I had to abandon Win98, then WinXP, and soon Win7 because of no working browser.
Linux is generally usable today, but as soon as Linus retires, it's going to fall into the same user-lockdown like all others. The basic infrastructure is all in place: secure boot, root login basically deprecated, access rights and security settings administered by the distro and package manager, not the user, no per-program firewall rules, automatic updates as standard, surveillance infrastructure via udev (hardware events), dbus (software events) and gtk accessibility (input events), etc. Linus fought hard to keep them outside the kernel, but he won't live forever.
To have any hope of privacy and/or freedom our personal computers we need to turn the security paradigm completely on it's head: users are not the threat - programs are. The user should login as root (or system account in Windows) and every program, including services, should run under their own limited accounts with no access to the rest of the system, especially user's files.
Of all OSs today, Gentoo portage is probably the easiest package manager to tweak into creating accounts and groups for programs instead of users and Gobo Linux has the best separation of programs in it's filesystem. I'd love to see a merger of these two.
Hobbyist computing? Ha! First get a browser working.