Ground fault detection
Your idea is not weird. The European practice is to have a whole house master GFCI (their term is RCD), but there are a few key differences.
- European homes' current demand is much lower than ours, because their voltage is double and their houses are less "electric".
- Their GFCI has a high 30ma trip threshold, which is good for protecting houses from burning down, but inadequate for life safety. A 6 or 8 milliamp trip is ideal for life safety, but if a whole house had that trip, it would have too many nuisance trips from tiny leaks adding up.
Arcing fault detection
AFCI needs to be per branch circuit. It depends on detecting the "sound" of arcing on the lines, and if it's protecting too many circuits, the sound is too dampened to hear.
Arcing literally is a sound; in fact it's the same "crinkle crunch" sound you hear when hooking up speakers or sticking headphones in a plug. If you hooked plain, dumb speakers wound for 120/230V mains power, you'd be blasted out by the normal 50/60Hz "hum". If you added a notch filter to block that, you'd literally hear line noise and arcing. This is easily analyzed by a 1990s era DSP.
Contemplate GFCIs for small subpanels
One cost-saver is to have the GFCI serve (or be the main breaker of) a small subpanel. However you don't want that serving too many circuits, or you'll have the European problem of minuscule ground faults stacking to cause nuisance trips.
You would not be able to do this strategy with AFCI breakers, because 2-pole AFCI breakers of large amperage are not made.
An alternative to AFCI protection is to run the entire circuit in metal conduit. The metal conduit conducts heat very well, so it distributes the heat from series arc faulting, preventing it from concentrating in one space enough to start a fire. As for parallel (shorting) arc faults, the metal conduit carries large amounts of current back to the breaker, assuring a trip. Since such a failure is also a ground fault, it will trip a GFCI quite a bit sooner.