What's this "still stuck on 2.5 inch" nonsense? Even for most high-end consumers (not prosumers running large CAD, video rendering/streaming, HPC programming, databases, or performance benchmarking), the speed of the Samsung 860 Pro, Crucial MX500, and whatever is Sandisk's latest are pretty much identical for OS loading, game loading, saving office documents, saving a screen capture or recording, etc., and they're all more than fast enough.

If you're on a thin&light ultrabook, or all you're going to have is 1 drive in your desktop system, then sure, make it an M.2 and save yourself the cables/bulk, but let's not kid ourselves. Most consumers will notice SSD vs. HDD, however most will NOT notice NVMe vs. SATA III.