The Mercedes weight distribution problem seemed weird at first, but I think I understand it better now.
The Mercedes problem was too much front weight which is weird because modern F1 cars carry most of the ballast at the front. You would think it’s no problem to move ballast rearward and presto!, you get the weight distribution you want. This is true in itself, but it's not the complete story.
If the overall C.G. location was originally designed to be 56% rear (made up number) then the fuel tank will be located so its C.G. is 56% rear. In this manner the overall weight distribution of the car doesn’t change as the fuel weight changes.
Then Mercedes goes pre-season testing and oops!, the tires actually prefer 58% rear weight. So you move ballast to get 58% rear weight, but the fuel tank is still at its 56% rear location. Now the fuel acts like front ballast that magically disappears during the race. If the car starts the race at 58% rear then maybe you’re at 59% rear by the end of the race.
Mercedes’ big-picture solution is to increase the wheelbase (~2 inch?) by moving the front wheels forward relative to the rest of the car. Now the same fuel cell is located at 58% rear within the wheelbase. Then they fine tune the ballast for 58% rear weight and go racing. The weight distribution stays at 58% as the fuel burns off.
F1 teams try to match the front-rear distribution of three things as closely as possible: tire grip, weight, and downforce. A good match makes the car faster and more consistent. Two inches or a couple % may not seem like much, but as others have pointed out we’re only talking about a 0.5 second deficiency over a 90 second lap.
I think the need to precisely match these 3 parameters also explains why teams are so stressed out over the prospect of completely new tires next year-- it’s a whole new opportunity to get it wrong. Confidence in downforce distribution will also be low next year because double-deck diffusers are gone.
How many teams will nail their estimates of all three parameters next year?
Perhaps a smart team will design next year’s car from the start with three sets of front wishbones to accommodate three wheelbases. Then they can quickly change the fuel tank C.G. to match whatever surprises come from the distribution of tires or downforce. This must be better than thrashing around during the season to change a car that wasn't designed to accomodate different wheelbases in the first place.