Steel, Railroad, and Industrialism are pre-requisites for Plastics anyway - so it's not an 'either / or' issue.
Plastics is at a point where it can be your second last technology (before Robotics) or it can come before literally over a dozen other technologies depending upon how we prioritise the tech's.
My general view is that if we go down this Plastics path, then we should really get going with it:
Plastics is a required technology (for Robotics and Docking Bay), and will also give a research discount on other required technologies in; Fibre Optics and Ecology.
If we choose not to prioritise Plastics, then it's hard to mount a case for The Three Gorges Dam.
I also acknowledge that even if we do prioritise this Wonder, we will be building some Coal Plants anyway - but maybe not 3050
worth (excl. multipliers).
It's up to the roster to decide - but I hope the decision is thought through.
Plastics is at a point where it can be your second last technology (before Robotics) or it can come before literally over a dozen other technologies depending upon how we prioritise the tech's.
My general view is that if we go down this Plastics path, then we should really get going with it:
If every city was to build a Coal Plant, that's 4,800
(excl. multipliers)
Cost of the Three Gorges Dam is 1750
(excl. multipliers) which provides less
than Coal Plants.

Cost of the Three Gorges Dam is 1750


Plastics is a required technology (for Robotics and Docking Bay), and will also give a research discount on other required technologies in; Fibre Optics and Ecology.
If we choose not to prioritise Plastics, then it's hard to mount a case for The Three Gorges Dam.
I also acknowledge that even if we do prioritise this Wonder, we will be building some Coal Plants anyway - but maybe not 3050

It's up to the roster to decide - but I hope the decision is thought through.