Another one of my questions where it seems a no brainer to me but others will likely see my ideas as silly. 
So here is my reasoning.
HDD's for the most part performance no longer matters anywhere near as much, it usually just needs to be fast enough to do things like play multimedia, or for maybe games. SSD's tend to take over i/o performance sensitive tasks.
2.5 inch drives are much quieter.
2.5 inch drives are much lighter.
2.5 inch drivers are much smaller.
2.5 inch drives run cooler.
2.5 inch drives take less storage space in a warehouse or delivery van.
Imagine the space used in a case if 3.5 drive bay was just 2.5 drive bay.
I am not sure if they cheaper to manufacturer, they definitely have a lot less material but they also may be harder to produce.
I am aware there is size/capacity constraints so what would be my solution? Simply taller drives. I have a 2.5inch WD drive that is thicker/taller than a normal 2.5 drive, e.g. it wont fit in a ps4, but you would get it in a PC drive bay.
Transitioning wouldnt be difficult as 3.5 bays typically can house 2.5 drives.
So I would stop manufacturing non enterprise 3.5 drives, and replace those with desktop 2.5 variants where they have extra height to cater for the extra spindles for 3+ tb sizes.
So here is my reasoning.
HDD's for the most part performance no longer matters anywhere near as much, it usually just needs to be fast enough to do things like play multimedia, or for maybe games. SSD's tend to take over i/o performance sensitive tasks.
2.5 inch drives are much quieter.
2.5 inch drives are much lighter.
2.5 inch drivers are much smaller.
2.5 inch drives run cooler.
2.5 inch drives take less storage space in a warehouse or delivery van.
Imagine the space used in a case if 3.5 drive bay was just 2.5 drive bay.
I am not sure if they cheaper to manufacturer, they definitely have a lot less material but they also may be harder to produce.
I am aware there is size/capacity constraints so what would be my solution? Simply taller drives. I have a 2.5inch WD drive that is thicker/taller than a normal 2.5 drive, e.g. it wont fit in a ps4, but you would get it in a PC drive bay.
Transitioning wouldnt be difficult as 3.5 bays typically can house 2.5 drives.
So I would stop manufacturing non enterprise 3.5 drives, and replace those with desktop 2.5 variants where they have extra height to cater for the extra spindles for 3+ tb sizes.