These are my reasons:
- Almost same resolution as the S1R
- Almost as good at high ISO and shadow lifting as the S5
- Smaller and lighter than the S1R
- Latest PDAF & fancy AI AF
- HHHR, Live Comp etc all in one camera
- Latest firmware etc with updates
- Access to Lumix Lab
- Better battery life than S1R
And maybe a few more...
I have not pre-ordered yet, I have Impatience sitting on my left shoulder fighting Sensible on the right

. I don't really neeeeed it now, but...
In my case it's such a wonderful upgrade from the S1:
- higher resolution at basically same DR, noise, rolling shutter etc.
- Smaller, lighter
- Same body and battery as the G9ii. Nice!
- PDAF
- Faster burst in mechanical and much faster in electronic shutter
- pre-burst
- HHHR
- Love the separation of photo/video/S&Q
- Same great EVF
I think the only thing I might miss are the illuminated buttons sometimes, the second front button next to the mount and the
Honestly, I've had the S1 since launch (almost 6y?) and it's still amazing. So the S1R will realistically cover all I need for the next 10y easily. Sure, maybe one day I might regret having "only" 9 fps mechanical when photographing hummingbirds in Costa Rica but what a first-world edge case problem to have....
Plus I will keep the S1 since the body has gone through a lot including repairs and for the low money I'd get from MPB, I much rather keep it. Nice to have two primes on two bodies in some situations
So yay! Excited!