Both make good arguments. If the total cost of an SSD is indeed cheaper than spinning rust in five years, then sure, I can see them getting phased out for enterprise.
But what about consumers? They don't consider TCO the same way. They don't look at energy or space constraints. They only look at cost per megabyte. As long as SSD is more, they will keep buying spinning rust for backups.
The fact that Costco always has backup disks on the shelves tells me that it's a pretty popular item amongst consumers.
I am a consumer, and I look not only at cost per megabyte. I also need to sleep at night while my laptop gets backed up and while my torrents get downloaded and seeded. Therefore, due to their noise, HDDs in the same room are not an option, no matter the price. For local backups and torrents (i.e. something that I can lose) I nowadays rely on an 8TB QLC SATA SSD connected to the router via USB adapter (in 2016, there was a 1.92 TB SSD), and will buy another one if it gets full.
I guess it's down to personal preference. The noise of data being written to a HDD is soothing like a lullaby to me. I can rest easy knowing that the hard drive is working away through the night, filling sector after sector with new surprises for me to find in the morning and carefully copying my old data keeping it safe and warm so that it's always there when I need it.
But what about consumers? They don't consider TCO the same way. They don't look at energy or space constraints. They only look at cost per megabyte. As long as SSD is more, they will keep buying spinning rust for backups.
The fact that Costco always has backup disks on the shelves tells me that it's a pretty popular item amongst consumers.