There are OEM requirements to keep and provide parts for a given number of years, usually 10 is what I here.
I own a 2006 H3, which had a number built, and I can share my insight from that. There are significant differences between comparing a truck like the Hummer (any model) to the Raptor.
You should not have any issues finding parts through a dealer netwrok for a number of years. I can easily get parts for my Hummer going through my local Cadillac dealer (remember there is not Hummer brand). I don't think finding OE components for repair will be difficult. I have seen more and more parts coming from China for OE replacements, my guess is that there are companies buying scrapped tools, refurbishing them, and offering parts through the internet.
I would expect the aftermarket to remain strong for these trucks. It is not as much the volume of vehicles built, but the market and attitude towards them. There are a number of companies supporting of vehicle models with low volumes. If the people want to buy they will.
Kaiser is right on the Hummer market. It was a poorly marketed plan for that truck, which was part of its demise, and there is very little aftermarket. Once the brand was killed, the aftermarket pulled out. I am curious to see if there is going to be a growth in that market (as used vehicle prices come down, will more people buy them with the intent to off-road them).
If I keep my truck.... I am not too concerned about getting parts 5 or 10 years from now.