you are thinking of resources the wrong way, sure they are using less actual steel but the game only tracks the value of steel
Those are the same things.
For example, if a railroad company wants to buy steel for replacing tracks, and they have the choice between steel with expected service lifetimes of 30 and 40 years, at equal price the second is 25% cheaper in lifetime costs. That means the cutoff for increased lifetime profit from using the latter is at 33% higher price than the former.
Of course that doesn't directly mean the second steel is actually purchased for 133% the cost, but all else equal there isn't a reason to purchase the worse product - giving a comparative advantage for the higher-quality steel, even if they raise prices a bit.
The net difference is that the actual steel amount traded is the same, but prices are higher per unit and more demand is met.
I don't see how that in game terms is any different than saying that the new process allows more steel to be produced per input of iron and whatever else the process requires - skilled labor, more time/less throughput (which the Open Hearth Furnace, PM #3, actually does compared to Bessemer), some other input good etc.
There are already some PMs that work like this (Electric & Diesel Engines for example, although they use a lot of fuels) but most of them seem to have increased base inputs in return for a proportionally smaller or equal amount of output.
In game terms, the difference is that a 10% increase in Steel per Iron unit means Iron consumption is reduced by 9%, meaning you need a lot less mines to run the Steel-heavy late-game.
If that's the cause. it's not really that coal is too expensive or people are too cheap. It is whatever makes it possible to apply a new technology across the board rather than selectively, way faster than it was done in history. Maybe resources that enable automation are actually too cheap. Maybe we're so used to a maximizing style of playing these game that we find it hard to adapt to a different one.
Yes. According to Wikipedia, Ruhr coal mines produced on average 120/t/employee in 1850 vs 179/t/employee in 1900. Yet the game increase of Coal productivity is from 8 units per 1000 employees at Prussian base techs (without railroad transport) to 28.3 per 1000 employees with Steam Donkey, Railroad Transport, Condensing Engine Pump and Dynamite.
But obviously there's still capacity left at that point historically, because total output almost doubles by 1913. Yet in V3 I've usually filled out every slot of everything in the mainland by 1870 and 15-20 years later I need to start conquering or the economy fails.