Funny you mention that because it has cost them at least one purchase. I was waiting for the z370 gtx to land for a couple of months. I even emailed Asrock, asking if they could ballpark a release, so I could plan for it. After enough waiting and no response from them, I finally built my own SFF PC, instead, and I'm pretty happy with the decision.
I think you made the right decision too, ironically unavailability of the Z370 variant played out in your favor. This is the problem isn't it, even if the product was available outside of Asia, it's not competitive with tiny ITX systems that aren't much larger. In the extreme case, something like the CustomMod Flex Mini at 4.5L has a full ITX board, fully internal PSU, and supports desktop graphics cards that are more powerful and much cheaper than MXM. Technically 4.5L is 50% larger than 2.7L, but it's still tiny and you're getting a lot more value and a fully custom desktop system. With a PicoPSU or G-Unique, you can get the case size down to 3.5L, though that is very niche. The S4M being the smallest mainstream option, is not much larger at 5L.
2.7L (excluding the brick) is just not ALL that much smaller than 3.5-4.5L to justify the limited functionality, limited power, limited upgradability, and most importantly the much more expensive and worse performing GPU. ASRock is getting the right idea with their new 2.2L slim variant -- I'm glad they've realized tiny size is all they have with this product, so making it as tiny as possible is worth it, certainly at the expense of 2.5" support. But it's still not worth it overall in my view.
You'd really have to go down to sub-1L, like a NUC, to say okay this is truly in a different size class. This is why Mini STX makes a lot of sense, only 1.5L with the Silverstone case, very affordable, and you're not really sacrificing on anything without a GPU. An AM4 Mini STX board would be amazing for an APU build. But MicroSTX is just not competitive with those larger-by-1-2L ITX cases. That's my 2 cents.
If ASRock manages to make MXM cards a truly consumer product and bring the price down to within 10% of PCIe cards, I will give them all the props in the world. They've already done a lot for SFF. Sorry I typed a lot, but I really find this subject interesting.