Hard drive density has stagnated. There haven't been any major technology breakthroughs since 750GB PMR drives came out in 2006. Most of the capacity improvements since then have come from minor materials improvements and stacking increasing amounts of platters per drive, which has reached its limit. The best drives we have, 24tb, have 10 platters, when drives in the 2000's only had 1-4 platters.

Meanwhile, semiconductors have been releasing new manufacturing processes every few years and haven't stopped.

Moore's Law somewhat held for hard drives up until 2010, but since then it has only been growing at a quarter of the rate.

Right now there are only 24TB HDDs, with 28TB enterprise options available with SMR. The big breakthrough maybe coming next year is HAMR, which would allow for 30tb drives. Meanwhile, 60TB 2.5"/e3.s SSDs are now pretty common in the enterprise space, with some niche 100TB ssds also available in that form factor.

I think if HAMR doesn't catch on fast enough, SSDs will start to outcompete HDDs on price per terabyte. We will likely see 16TB M.2 Ssds very soon. Street prices for m.2 drives are currently $45/TB compared to $14/TB for HDDs. Only a 3:1 advantage, or less than 4 years in Moore's Law terms.

Many enterprise customers have already switched over to SSDs after considering speed, density, and power, so if HDDs don't keep up on price, there won't be any reason to choose them over SSDs.

sources: https://youtu.be/3l2lCsWr39A https://www.tomshardware.com/pc-components/hdds/seagates-mozaic-3-hamr-platform-targets-30tb-hdds-and-beyond