See, displays !
My answers, not GHIs.
1) G400-S is unrelated to this Gadgeteer announcement. GHI have many other commercial customers and have really good product end-of-supply processes, so I think you'd have heard about that if there was any reason for concern.
2) GHI have said they continue to support Netmf. What that means is that they're not shying away from revising things as needed. They haven't announced any other plans (and I suspect will not) about future SDK releases, but there's nothing to suggest 2016R1 is the last NETMF SDK you'll see, just the last one with Gadgeteer included
3) Displays! Yeah I personally LOVE the flexibility that the Gadgeteer sockets bring for displays. I suspect GHI will stop producing T43, but the NHVN one may be simpler to produce, but there's still not a high demand. There is also the 4.3", 5", and 7" Newhaven PCBs that @ Dave McLaughlin has released with his company's branding that could be an option for you
4) Only you can evaluate that, but there's no sign of GHI giving up on the platform as a whole; diversifying yes, but for my money there's plenty of reasons to be here in the first place that won't change
@ Darren SFI,
J11 is a specific beast I think, but it's design isn't hard to replicate either. Same for RotaryH1. GHI haven't said what if anything they intend to do with design files and gerbers, perhaps an appropriate ask is where possible that they release those to the community as well. And if you're going to use G120, then you are designing custom PCB, easy enough to add most of the capability onto your design in the first place.