Perhaps, but today, it's not.
A modern HDD might do 150MB/s on a sustained transfer today, while on SATA2 an SSD might hit 250MB/s. A decent boost, but not substantial. Even on SATA3, you're still talking about maxing out at what, 500MB/s? This is not even an order of magnitude difference, it's tiny. It is, perhaps, 3x faster.
On random reads, on the other hand, you're talking about the difference between 0.2MB/s and 200MB/s. That's three orders of magnitude, or 1000x faster.
Which do you think is going to make a bigger difference, a 3x increase in sequential read speeds, or a 1000x increase in random access read speeds?
It's also silly talking about maxing out SATA bandwidth with a 20TB SSD... By the time we have affordable 20TB SSDs (by Moore's law, that'd be about a decade) we won't still be using them on SATA3.