Promethenium science repeatables

[Dictator]

TL;DR
All machines should be able to reach its productivity limit at huge material costs.

What?
Some recipes have infinite repeatable, that increase their productivity.
With use of legendary productivity 3 modules you would have to spend 10M - 100M promethenium science total.
If you wanted to get rid of all productivity modules while maintaining maxed out productivity you would have to spend much more on it.

Why?
Currently promethenium is fairly useless - used only to increase research productivity.
With it you would be able to increase your SPM in your downstream supply, as you are producing up to 4x more resources from same input.

[CyberCider]

Yeah, promethium is so useless, it can only research the most powerful infinite technology in the game

[Dictator]

I know, its very good bonus.
But since its post endgame content, why not just have ability to increase productivity for all recipes.