I figured out that there are two types of people: some prefer to use the original old power supplies, the others take modern ATX power supplies and use them with adapters. There are problems though:

1. Original old power supplies can fail. I know some of them are very reliable, but some components can give up due to either old age or their length of service. And when an old PSU dies, you just pray that it doesn't take a something more valuable with it.
2. Original old power supplies are inefficient. They can be very big, hot, noisy and their estimated efficiency is 80% at best and 60% at worst. Some would argue that's part of their charm.
3. Modern ATX power supplies are not designed to power retro systems. They lack -5V rail (not a very big deal: not all systems use it, and it can be added with a simple mod), and their +5V rail is generally very weak.
4. Modern ATX power supplies can fail too, especially when they are used outside of their spec. Generally, when connecting such power supply to a retro system, +5V rail will be overloaded, and +12V rail will be underloaded, and they are not designed to work in such unbalanced setup. Best case scenario: the power supply would just shut off, worst case scenario: the PSU dies, and you just pray again.

Is there a modern PSU design for retro computers? Using modern components like GaN FETs, it's possible to create a very small and efficient design specifically tailored to power retro computers. Did nobody make one yet?

I understand that there are some challenges. First: mains is different around the world, Japan has 100V, and Kuwait has 240V, all other countries have everything in between, some are 50Hz, others are 60Hz, and the outlets are all different shapes. Solution: no mains input, we expect clean 12V DC on a barrel jack, and a 12V brick power adapter is pretty easy to get locally in almost any country.

Second: one size won't fit all, retro computers from different eras have different power requirements. Solution: make a modular design. I have made this table of typical current on every rail of a PSU:

1+-------------------------+------+-----+-------+------+------+-------+
2| Typical computer        | +12V | +5V | +3.3V | -12V | -5V  | 5VSB  |
3+-------------------------+------+-----+-------+------+------+-------+
4| XT, AT                  | 0.5A | 10A |       | 0.3A | 0.5A |       |
5| 386, 486                | 1A   | 15A |       | 0.3A | 0.5A |       |
6| Pentium, Pentium II     | 3A   | 20A | 7.5A  | 0.3A | 0.5A | 0.75A |
7| Pentium III, Athlon     | 6A   | 15A | 15A   | 0.3A |      | 1A    |
8| Pentium 4, Athlon XP/64 | 12A  | 10A | 20A   | 0.3A |      | 2A    |
9+-------------------------+------+-----+-------+------+------+-------+

The biggest task for this power supply would be 12V to 5V step down, and such modules can come at just 3 different options: 10A (XT, AT, Pentium 4, Athlon XP/64), 15A (386, 486, Pentium III, Athlon), 20A (Pentium, Pentium II). Other modules are similar.

Final very important note: protection. Every PSU needs to have good fuses, and in case something goes wrong, shut down gracefully without damaging the system that it is powering.

I am not a power supply designer, and I don't have any experience designing circuits with GaN FETs, but to me it seems like there must be some demand for a project like this. Anyone working on something similar? Or maybe you know why my idea is doomed to fail? Any input is appreciated.