I see SATA M.2 sales declining, but not entirely going away. There will probably always be a brand-name vendor making SATA M.2 SSDs, at least for the next 5 or so years, I'd guess. Beyond that, who knows? Most M.2 SSDs these days, at least the ones that are "sexy", are all PCI-E NVMe, and pushing crazy sequential speeds, which barely matter in the real world for most people. But they have "big numbers" when benchmarked. (Why else did Samsung enable RAPID mode by default for SATA 2.5" SSDs?)
So, yeah, SATA M.2 has pretty much all of the disadvantages of SATA, while taking up a precious M.2 socket. Problem is, some laptops sold in the past, and probably today, have SATA M.2 sockets, that aren't also PCI-E compatible, so there will still be a market for SATA M.2, I feel. At least, in the near term.
Edit: There is also generally a cost advantage for larger M.2 SSDs, between SATA and PCI-E NVMe, although with the advent of cheaper controllers, that difference is eroding. For those people with desktop motherboards, with MULTIPLE M.2 sockets, a smaller PCI-E NVMe M.2 drive for OS, and then a larger, cheaper, SATA M.2 drive for bulk storage, also makes a lot of sense.
Although, in five years, they will probably all be QLC 3D NAND, 128 Layer, but who cares, right, it's just SATA6G transfer rates anyways.
The real future, is "3DXpoint NVDIMMs". Slotted in like DRAM, using DRAM-controller protocols, but persistent like NAND flash in SSDs. No more "loading" of apps, everything on the PC is "instant on".
How long that tech takes to trickle down to consumer PCs and laptops, remains to be seen. The OS (Windows and Linux and whatnot), will have to be modified, too, for that "instant on" paradigm.
Does your current laptop, have a HDD or an SSD? If it has a HDD, then don't wait, get an SSD slapped in there ASAP. Although, if you want to wait, NAND is in oversupply right now, and prices are predicted to drop by 50% (per bit, which is another way of saying, moving to QLC soon) in 2019.