Thanks for the correction, what specifically makes them dead end that we can't re-use the designs - e.g. use modern parts?
Magnox: un-enriched fuel, designed for weapons, low operating temperature.
AGR: too complicated, can't be refuelled without complete shutdown.
PWR has emerged as the only viable design, and even that is too complicated compared to solar+batteries.
Basically, plumbing is the enemy. Every joint is a potential failure. If what's in the pipe is radioactive or corrosive or explosive (in the case of molten sodium, all three), that's extra bad. So there's tremendous cost of inspections.
Or you go for a gas. But then you're not able to use water as a moderator, so it either has to be a "fast neutron" design (which the few liquid sodium reactors are) or use a graphite moderator (flammable, lifespan issues with neutron cracking).
The unexplored corner is https://en.wikipedia.org/wiki/Gas-cooled_fast_reactor : plumbing is "safe" apart from being incredibly hot, but you still have to deal with fast neutrons smashing up the reactor from the inside on an atomic level.
CANDU works fine too.