Tape and disk cost a *very* similar amount per megabyte now. There are capacity points where things swing one way or the other, depending upon controllers needed and so on, but disk is pretty close.
Until you realize that tape is much less reliable, which means that tape backup requires multiple storage passes. In addition, the extremely poor access times of tape mean that you'll have to back up more data more often, which means the total capacity used for tape will be higher than disk backup.
Tape is now more expensive than disk.
This doesn't even get into the massive savings you get from the managability and performance of disk, so you don't have to spend (so much) money managing the physical requirements of the storage mechanism.
The only advantage of tape is that it's easier to move around in situations where you might drop it. This means it's easier moving data offsite or for protecting it from things like vigorous earthquakes.
But yes, it took decades for tape to fall off the cliff more or less entirely. Disk will be the same way.