system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k] The Imperium finds a STC for all patterns of Terminator armor. How does this change the Space Marine chapters?
gpt
Something to remember about the 40k universe is that the Imperium has some fantastic technology, but it's distribution is incredibly limited because of the scale of the Imperium.  There are likely individual tech Magos that understand Terminator armor (and may oversee Terminator armor production lines) that could craft a Chapter Master a suit of armor worthy of a Praetorian from the Great Crusade... but it would be a side project for him, and he lacks the production capacity, materials, and tooling to mass produce them.

To spin up production of a new pattern of Terminator armor, a Forgeworld would need to ensure several things: 

* a reliable supply of materials (some likely rare or nearly impossible to find without more expenditure on arcane mining equipment, which requires new production lines of its own...)

* trade contracts with the various entities that own those resources, control trade routes, or own interests in various property

* real estate on a Forge World to set up the production line

* enough skilled tech adepts able to learn the patterns, specifications, tolerances, operation of new equipment, and quality control

* enough raw labor to fill all the 'unskilled labor' portions of production

* patrons willing to protect them for the time it takes to procure all these resources and land

* Space Marine chapters able to offer valuable services, protection, or loyalty pacts, or technology to 'trade' for the new armor

* assurances that they could produce enough armor over the next several centuries to make it worth the investment instead of expanding the new Auto Boltrifle line that Mars is pushing them to produce for all the new Primaris marines (and that the chapters are begging for)

The STC probably exists out there somewhere, but the politics of the Imperium are preventing it's full use.