Well, at least Nikon and Canon are proposing some clearly unique lenses and their mount characteristic has the potential to help design better optics. In the end it's all about lenses.
Yes, it is all lenses... I don't see bad start options from Panasonic. With difference of Canon and Nikon they can not use adapted lenses from MFT (well, they can, but with reduced resolution if they make an adapter). Panasonic is releasing some typical and very used zooms and a prime, it is a good start for those that can not afford Leica alternatives.
From an AF standpoint, I have never heard great things about the GH5 be it for still or video.
At least they do have IBIS, which is a good thing obviously.
Yes, as I commented the AF could be the weak point in Panasonic offering. It is my understanding that firmware updates improved the AF system in the GH5 (since it is all done in software), but not sure if it is enough to make a Contrast AF system as quick as a PDAF system or as smooth as Canon Dual Pixel...
Even if you start from scratch with no existing lens investment in any system, how does the past track record of Panasonic in the still segment motivate you to invest a large amount in this system over Sony, Nikon or Canon?
Well, starting from scratch I will be carefully considering buying any of Canon, Nikon or Panasonic new systems. I didn't jump ship to Sony mirrorless system until I saw that they have natively all possible lenses that I wanted to use.
Panasonic is very well loved for video people, I think they are going to first make GH5 and G9 owners to think to jump FF with their system, instead to going to something like an Sony A7s and them, building the system to attract more users to it. I don't think they will be trying to become market leaders.
Also, don't forget Sigma... They announced that they plan to release cameras for this system, it will be interesting to have lenses that can be used with a foveon type camera.