[2019] The real problem with fluids is the discrete vs continuous mismatch between flow and buildings.

Right now, crafting buildings work rather unrealistically - instantly creating in a single frame a large amount of fluid and then nothing for many more frames. In the real world, things produce and consume fluids at rates. This is fine for items as they are discrete on belts, but fluids are not discrete in pipes.

If buildings consumed and output fluids at continuous constant rates based on their crafting speed and productivity (the fluid could still be produced in chunks, as long as those chunks don't actually interact with the fluid system directly) then - if a factory is well designed - the fluid systems in it can converge towards and reach a steady state. A state where you are recomputing the exact same flow state every frame, and so do not need to recompute it at all.

The whole connected fluid system can then go to sleep until some outside factor disturbs it, costing no further UPS unless the player messes up somehow.

It is, in essence, the best of both worlds - the no UPS cost aspect of electricity style water, but based wholly on clever optimization rather than changing the flow mechanics. It's even more realistic than the current system, rather than less.

Interestingly, to make the most of it for electricity, solar would have to be lower priority than nuclear, so that the flow rates don't change as power consumption fluctuates.

Addendum: In theory, with a sufficiently advanced factory analyser, you can do the same with the discrete items in simple enough factories (circuits are Turing Complete, so no go there), and so remove functionally all UPS limits on factory size on biterless maps. But analysing a discrete system like that is radically harder than the simple checks you can do for the continuous fluids.