Yeah, that's certainly true (not to take anything away from the GH5 though).
The other thing is I don't want to have to buy (and carry around) two sets of lenses: one for FF and one for M43. I understand that using two different sensor sizes allows a single full frame lens to effectively become two different focal lengths depending on to which camera it is mounted, but I am not sure how much of a benefit that is.
And if I add the cost of a decent speedbooster, it isn't all that much cheaper than a used S1 (going about $1700 with VLOG upgrade).
I understand what you are saying, but I think I would probably WANT a speedbooster on a GH5... but then again, not sure.
On the other hand...
The original plan was that I would just put the B Cam on sticks and let it roll during interviews / corporate work / events. But I have an unruly teenage son and so now the misses wants me to start taking him along on gigs too so that he doesn't end up working in a gas station. I think he would be much more comfortable with a GH5 / G9 than with an S1 with heavy metal lenses on it.