>It's not like they weren't producing plate armor in the early middle ages because it cost too much. They weren't producing it because they couldn't.

Not really, they could had taken smaller plates and forge welded them together into a larger plate by hand. At the extreme, you have smiths in 5th century India hand forging smaller pieces of iron into a 6 tonne iron pillar (see Iron Pillar of Delhi). But of course, doing it this way cost a premium in labour and fuel.