This was just a small observation from WD304. Basically it states that the AM can still manufacture Terminator armour, just in very small quantities due to the level of skill and effort required. This is a recurring question that has been debated at people ponder exactly how much tech the Imperium has access to.

It looks like Terminator armour is not completely irreplacable but it is very hard to make. Possibly the same is true of other exotic items like Dreadnoughts etc.