The reasaon for the slowdown is because the problem has stopped being about engineering and is now about physics. Sectors have become so tight onto HDD it's getting finicky to make them work and keep writting and reading and stuff like EM interference and even cosmic rays are playing a part.
It's why tapes have had a comeback with LTO format, as the advances of HDD engineering can be applied to tapes but weren't till now. Sadly tapes are pretty slow due to again physics as you can only spin a strip of plastic with chemicals on it so fast before you strat risking it tearing the fuck up.
HDDs have hit a wall of physics, not of engineering. Well technically it still is engineering but the issues are ones that require new better understanding of physics to solve.
Me I am hype for the LTO price crashing on the next few years.