What effect do the bargeboards have exactly that will cost teams 3 seconds of lap time? They are more than flow conditioners, they certainly generate downforce on their own. In fact that is how they are so effective at flow conditioning. If you look at them, they don't look like aerofoils the boomerangs seem to be creating lift, however one thing is clear. The general shape of the bargeboards forces air to flow faster underneath the car than over and around the bargeboards. Air has to outwash over the bargeboards, while under and around the periphery, the air flows much faster. Speed differential = pressure differential, they are essentially ground effect wings, combined with the side pod end fences they have enough surface area to match either front or rear wing, and possibly the diffuser.
By removing this piece from the equation teams will have to rely on only the front wing, rear wing, and diffuser. In essence the new regulations remove a wing from the car while making the main one(the floor) bigger and more effective. The question is how much freedom do they have in that area? One of the reason a manufacturer deal is so essential to victory is that the rear end of the car(transmission, power unit, ancillaries, fuel cell) all take up downforce disturbing volume. If you are the engine and chassis manufacturer you can tailor all the rear end parts to take up as little space as possible freeing up more critical downforce producing real estate. Furthermore you have a bigger budget, and more resources to make this happen.
One of the reasons the field closed up in the late V8 era was because the diffuser had been made smaller, that and blown diffusers meant that the diffuser didn't make as big a difference as other areas of the car. So the rich big teams which could increase the volume of their diffuser due to better packaging lost out.
This year and last, Mercedes made strides in their downforce because they kept squeezing more space in the rear end of the car. This gives the diffuser a huge performance advantage, especially these large diffusers that have the potential to generate incredible downforce but are hampered by bodywork directly in front of it.
Will this change going forward into 2021? Probably not.