As long as it came with costs, I wouldn't mind this at all.
The problem, though, is how to model the costs. In Vic2, the cost would iron and coal. If there's no iron or coal for a country to import (as many poor steel producers in HOI4 would face), they will pay inflated costs for what little iron and coal they can find, and may not be able to fully run the factories.
How would you model that kind of cost in HOI4? If you just do it with factory slots, everyone will just build up the required steel factories, and then Oprah Winfrey is going to give everyone steel.
I don't see why simply removing (or greatly reducing) steel cost for infantry equipment isn't a viable solution.
Planning around what resources you have is part of the fun, so allowing everyone infinite steel production definitely isn't a good solution. If it becomes as easy as is acquiring rubber and oil now, it would be bad.
Infantry equipment should be available to anyone with with IC to build it. Instead of going 2/3/4, the steel cost could go 0/1/2? Only a few non major countries at the moment could afford 5 MIC producing infantry equipment 3. Off the top of my head, only (unified) China and India.
Entire South America produces around 40 steel. That's 10 MIC producing infantry equipment, with closed economy. With export, only 5 MIC. And nothing else needing steel.
I think I'll try a game with steel cost removed from infantry equipment, just to see how AI performs.