A heap of bad news?

I think it really depends on the size of the system.   Say a smaller system like Samsung NX was cancelled.  You'd still be able to buy up stock for a couple years while it lasts, but then there isn't really a huge active user base to support long term used sales.

If one of the m4/3 players dropped out, there are more.   And even if all production was ceased on all m4/3 products, there is a large user base and a large collection of glass to keep it going for at least a decade.  Sure you won't get any advancements, but that doesn't mean it can't be an excellent system for a long time.

One of the big lens mounts like Canon, Alpha, Nikon would take decades to really kill, even if the company went out of business.  And surely someone would continue to make adapters because that would be a big market (80 million EF lenses, 70 million F mount, 30+ years of Minolta mount, etc).

m42 mount died in the 60s and people have been adapting those to newer cameras ever since.  If you have a killer set of Takumars, you could still be using them.