How have they doubled their prices?

https://www.pcworld.com/article/2642674/plexs-lifetime-subscription-plan-is-getting-a-massive-price-hike.html

HDD manufacturers have been increasing their prices as of late for the same products that were previously cheaper

Nope. Most recent delta is a decrease of $0.0000022 per MB. And the general trend has been decreasing for decades.

my users can watch on their mobile devices for free using a license I paid for once close to a decade ago.

You sound like a boomer talking about the house they bought decades ago for $75,000.