From a more zoomed out look, you could simplify it to only two speeds. The fast speed taking 0-15 seconds, and the slow speed taking minutes to hours. Any content that's been accessed once or twice in the last week or month would be on the normal tiers. Extremely dead content could fall to the tape tier, but it has nowhere further to fall, and it would take only a tiny amount of activity to rescue it.
I don't really see a reason for there to be a continuous falloff in speed. There's not really anything between hard drives and tape for responsiveness, either existing or proposed, that I'm aware of. Nor is there anything slower than tapes.
It makes me wonder what a storage device would look like that is cheaper than HDD, similar or better storage density, and allows random access, with a trade-off for slower speed?
The next immediate step from online tapes today could be offline tapes with online indexes & robotic retrieval systems. These exist today. The continuous falloff would be a matter of priority ranking given to content requests-- not merely FIFO-- so ever less popular irrelevant content gets shoved further back in the robotic retrieval queue. A recently iced bit of content might be top priority for the tape loader while something not touched in years might sit hours down the queue. The continuous decline isn't defined by the storage media but instead by the capacity of the retrieval systems. Speed would continue on a slow decline as content increases even more and the low economic value of that content make investing in increased capacity impractical.
Eventually you get to a point in some far off future where the retrieval time for some obscure bucket of bits is measured in significant fractions of a human lifetime, where a dying grandfather requests a video of his wedding 70 years earlier only to have it arrive just in time for his own grandson's dying moments decades later.
I think I've gone too far imagining unlikely slow storage dystopian futures though, so I'm going to stop now before I start ranting about the Slow God who needs only enough access requests from the masses of his adherents to prioritize his retrieval from the depths of cold storage. But Dante Alighieri warned of what was stored in the coldest depths and it was no god... Oh God what hath this comment awakened?!?!
Okay now I'm really done.
And I'm already assuming the tapes are offline, because online tapes would just be a waste of money.
Another issue is finding enough content suitable for very high latency systems. Right now they seem to basically just be for backups.
It's also quite telling how in most of these services, old content is very well hidden in the UI and sometimes near impossible to get to.