Everyone is going to buy M.2 SSDs first, and only buy SATA if they don't have enough M.2 slots. I really doubt SATA SSDs are selling well.
With that said, I don't see SATA going anywhere. It's (comparatively low) bandwidth means you can throw a few ports on your board and not sacrifice much. For some quick math: a M.2 port back-hauled by PCIe 4.0 x4 has 7.8 GB/s of data lines going to it. While SATA 6.0 has only 0.75 GB/s of data lines going to it.