I replaced this today, in continous service since 2017. Platter cannot spin at all, heads will not move and will not park. No scratches on disk surface from the heads and heads look fine under the microscope. Is this simply an electronics failure where the head can no longer move and thats what caused the death ?

I've seen this happen multiple times on some drives I own. What you did (spinning the platters with the heads on) is the worst you can do as it can damage the platters. What I usually do is slowly move the heads towards the parking zone (in your case the side ramp) while making sure no fingerprint goes on the platters, and the disks often goes back to normal. The only problem is that they often get or develop bad sectors afterwards (if they start working at all) because they got opened. Doesn't seem to me like the electronics of the drive are bad. Just seems like the heads were stuck and the sound the drive made when powered on was the drive trying to unstuck them.

I don’t know why but my first thought was to think that it had a good beat

In a former life id be called on to retrieve data from DOA HDD's so I had a cabinet full of different drives to borrow controller boards from to retrieve the Data or do a raw read of the physical drive had crashed. Physical crashes were rare which made the task way quicker and as simple board swap would usually make the data available again with IDE drives, SCSI drives were a bit more work as they were sometimes more complex.

Good