A breakout high dynamic range 50+ megapixies mirrorless full frame from Sony that worked with all Leica & Zeiss M mount lenses. I've rancid high pressure GAS for that.
I'm pretty happy with m43s, overall. It suits me. M43s competitors are getting close when it comes to body size & weight, but lens size & weight is still problematic.
That is true but f6.3 on a Full Frame is equivalent to a much faster lens on an M43 lens when it comes to keeping ISO and noise under control. This is one area where equivalence comes in to play. This is far less relevant in good light.
Nothing really, I change my cameras when I wear them out, or they break beyond economic repair. If I wear out or break my current cameras I'd probably replace them with the same model as I'm pretty happy with what I have.