I think neither are mainstream configurations, the only people I know off trying to use m.2 as mass storage is people on tech forums, especially techpowerup.
If motherboards only ever served mainstream use we would only have 1 pci express slot, 2 sata ports, one m.2 port, micro atx form factor, 2 dimm slots, no overclocking chipsets, low end VRM, and only basic bios options.
Dont kid yourself people using 4+ m.2 devices is some kind of mainstream configuration it is not.
However the difference with sata ports is they have a tiny footprint, making a board have 6 or 8 instead of 2 isnt at the expense of removing other connectivity, its only downside is making a board a few dollars more expensive. In other words it didnt compromise motherboards.
M.2 slots on the other hand go in place of PCIE slots and significantly compromise the versatility of what a motherboard can do and its expandability options, they force a compromise on to board designers and ultimately on the consumers buying these boards. With 2 of them on a board it was just about possible to not lose anything but more then that they been placed in locations that compromise the board. It makes a mockery of the PCIE lane advancements that have been made, in the past the issue was a DMI bottlenecking capacity, now its a lack of PCIE slots.
There is solutions that could make everyone happy, vertical mounted M.2, M.2 PCIE addon cards, back mounted M.2 (would require new case designs), U.2.
So yes you may prefer M.2 as a replacement mass storage, but dont pretend its a mainstream configuration. On places like reddit and mass storage communities, there is a fair amount of people using storage addon cards to expand functionality, so you find the relevant people in the relevant communities. Streaming communities capture cards etc.
couldnt hold back...
