An observation:
The GX8 and the PEN-F were both rather expensive enthusiast cameras and they are both gone without being replaced. The alternatives are either larger, more advanced cameras like the G9 and E-M1, or cheaper, less advanced alternatives like the GX9 and the E-M10.
Both of those categories have their place. The cheap ones because they are cheap and can be used with the tiny prime and zoom lenses made for MFT, the advanced ones because there are enthusiasts and professionals in need of equipment that is much more compact than the full frame alternatives. Making compact full frame cameras is no problem, professional grade lenses not so easy.
The middle ground however, is more complicated. Take the Canon RP. When compared to other full frame mirrorless cameras, it's not much to brag about. Compared to a GX8, which is more or less the same size, it's a different story. It's a much better camera, plain and simple, for a price very close to what the GX8 originally sold for. The PEN-F is of course much smaller, but not much less expensive, and it kind of falls into the same trap.
Later, there will be a Nikon Z5 or something, and Sony already offers cheapish alternatives to these cameras.
My MFT setup now consists of small primes (8, 12, 25, 45... 75 will be added) and a long telephoto zoom (100-300). This is what MFT is best for. I'll use the GX8 bodies till they break. Then they'll be replaced by smaller bodies and/or bodies more specialised for video, like GX9 plus G95. In addition, I'll buy a simple full frame setup for occasions when that kind of quality is needed, most probably Nikon or Canon, since that gives me the possibility to work with digital and film using the same lenses.
I don't think MFT will die, but some niches will disappear. My prediction is that they will mainly be left with small/cheap (GX9/E-M10), compact enthusiast (E-M1/G9 plus E-M5/G95 for those with less money) and professional video (GH5/Blackmagic/Z-Cam etc.). I also think that Olympus will enter the full frame market. Rationally, they should use an existing mount, but knowing Olympus, they'll probably invent their own.