A lot of folks have brought up the Geissele rails non-compatibility with "standard" attachment systems, however not a lot of folks have really been able to answer for me what exactly it is that they want to mount to the Geissele that cannot already be done with a little bit of ingenuity.
So far, I've attached lights, bipods, handstops, VFGs, even the TNVC TAPS, all directly to the SMR, sling mounts are built in (though I recognize that some might prefer other options or locations). Others have attached covers and other items to them without much problem as well. To be fair, some of those items required simple modifications, but never more than what could accomplished with simple hand tools and perhaps a trip to the hardware store for extra screws.
I also say this as someone who has long since stated, and believes that full rails are still the most practical option for military weapons, and I'm not necessarily "against" an M-LOK version of the rail being made or anyone preferring it, I'm honestly just trying to think of what items it is that folks want to direct mount to the handguard that can't already that is available in a KeyMod or M-LOK configuration. I suppose if you want a particular brand of something, that might be an issue and it might be made in M-LOK or KeyMod only.
Beyond that, and having handled Geissele's MK 8 rails, as well as the CSASS mock-up, I love Geissele stuff, so don't misinterpret this as a slam--but I do feel that the M-LOK rails are ergonomically inferior to the curved cross-section of the current SMR/MK 1 format rails, due to the flat planes that are required for both KeyMod and M-LOK. This is, of course, obviously a very personal opinion, but I do think that the 416 SMR fits and feels much better in the hand than the M-LOK handguard on the CSASS.
Finally, I don't pretend to have any inside knowledge of the exact requirements for which the 416 SMR was designed or why the requirements were what they were, but I believe that it was specified that it should have "hard" mounting points for rail sections, and not a "negative" mounting footprint--while KeyMod and M-LOK might not have been around, PCAPs, VTAC, and other mounting systems did already exist. Moreover, Geissele just recently did a custom run of SR-25 handguards for "somebody" that specified a MK 1-style attachment system, despite the fact that they were already producing M-LOK rails.
Given the fact that Geissele's HK rail production and availability seems to be almost entirely driven by contract production, and the fact that "someone" still prefers the MK 1-style attachment system, and given how small the commercial HK aftermarket handguard market is, I don't know that I necessarily see them making an M-LOK 416 rail, considering that the CSASS and HK416 SMR are being driven by completely different customers with completely different requirements, with little to no overlap in platforms.
IMHO, your best bet of seeing an M-LOK 416 SMR might be if and when the Marine Corps decides they might try to upgrade the M27 and its handguard, and even then, there's no guarantee that it would be an M-LOK interface.
Again, I love Geissele stuff, but considering that they pretty much have said that they don't even bother to do commercial runs of the 416 SMR until they have a contract order, at which time they make some for commercial sale as well, and the huge number of other projects, requirements, and products that they have, I just don't see them investing in the tens of dozens of M-LOK 416 SMRs they might eventually sell without a contract requirement coming down for one.
But, then again, I've been wrong before. Just the sense that I get at the moment. Maybe if HK really did ramp up domestic production of an MR556, and it became a much more common weapon on the commercial market, things might change. Not holding my breath, though, and as I said, even if there was an M-LOK version available, I think I'd still prefer the "OG" 416 SMR, if only for the superior "hand-feel."