I don't know the area of a typical car radiator either, but I'm betting
it is much more than 3-4 square feet.

Yes, car radiators are much larger than 3 or 4 square feet. Car radiators
have to sit in traffic, not moving, in 110 degrees. Also, they have even
hotter air blowing past them, from the AC radiator in front of it.
Automatic transmission cars also have to cool the tranny fluid, by running
it through the radiator. That represents a huge amount of heat.

Most water cooled homebuilts do not tolerate sitting on the taxiway more
than a few minutes. How do I know that? By talking to the owners of them,
when they are deciding about when to taxi, and how long they can stand it.
There are exceptions; some have figured out how to cool their engines,
sitting on the ground for a long time. They are the exception, in the
homebuilt world. So really, that is how they get away with such small
radiators. Lots of cool fast moving air.

Also, keep in mind that you only
need to de-ice the leading edge of the wing (look at how much boots
cover).

Different princples at work. If you *melted* the first few inches of ice,
it would run back and re-freeze, before it got off the wing. Bad thing.
VERY bad thing, as in guaranteed crash, after about 1/4" of ice.

So the area is probably closer to 2' times the wingspan which
means 50-80 sq. ft. one a typical light airplane.

Nope, see above. Good for crashing, if it did stay hot enough to melt ice.

And you don't need to coolant at 180F+ to deice a wing, so you can push
the coolant through a lot more area than a radiator and still keep
sufficient temperature to melt or prevent ice.

You will have to add another pump (more weight) to move that "cooler" water,
then. ALL of the water that comes out of the engine is at 180 degrees.

Preventing it is easier than melting an accumulation to be sure. This
holds with other systems such as TKS as well.

Different, once again. Preventing it from forming on the leading edges, and
letting it freeze further back does no good. TKS keeps everything that runs
back from freezing, because the antifreez has mixed with all of the water
running back, to make a mixture that is above freezing.

If you melt it at the front of the wing with heat, the water must stay hot
enough to run ALL OF THE WAY back, and off of the wing. Jet engines have
enough waste heat to do that. IC engines DO NOT. Get that through your
head! It is not done, because it CAN NOT be done! Get it?

For my final argument, why are wing radiators not commonplace?

I'd guess cost and complexity would be two big reasons.

Yes, but you missed the biggest one. Weight. Weight. Weight. If it were
cost and complexity, someone would still do it. They *will not* sacrifice
the weight.
--
Jim in NC