Yeah, stuff like where the direction of flow depends on the order you've built the pipes isn't realistic at all.

I'm looking forward to try the new mechanics and I don't think the new system will make trains obsolete. Using tons of pipes will create large buffers, which will have low throughput until the pipes are filled sufficiently. You might still need to build pumps regularly to prevent backflow into large buffers, e.g. to not cause huge backflow into the entry pipes when connecting another new oil pumping outpost.

They could also add other mechanics for balance, for example if a pipe gets destroyed that could empty out the whole associated buffer (as the fluid would've leaked out), again making huge pipe networks dangerous.