Lets assume you truly believe the difficulty is the heat transport to the radiator, how is it solved on earth?
It's both. You have to spread a lot of heat very evenly over a very large surface area. This makes a big, high-mass structure.
> how is it solved on earth?
We pump fluids (including air) around to move large amounts of heat both on Earth and in space. The problem is, in space, you need to pump them much further and cover larger areas, because they only way the heat leaves the system is radiation. As a result, you end up proposing a system that is larger than the cooling tower for many nuclear power plants on Earth to move 1/5th of the energy.
The problem is, pumping fluids in space around has 3 ways it sucks compared to Earth:
1. Managing fluids in space is a pain.
2. We have to pump fluids much longer distances to cover the large area of radiators. So the systems tend to get orders of magnitude physically larger. In practice, this means we need to pump a lot more fluid, too, to keep a larger thing close to isothermal.
3. The mass of fluids and all their hardware matters more in space. Even if launch gets cheaper, this will still be true compared to Earth.
I explained this all to you 15 hours ago:
> If this wasn't a concern, you could fly a big inflated-and-then-rigidized structure and getting lots of area wouldn't be scary. But since you need to think about circulating fluids and actively conducting heat this is much less pleasant.
You may notice that the areas, etc, we come up with here to reject 70kW are similar to those of the ISS's EATCS, which rejects 70kW using white-colored radiators and ammonia loops. Despite the use of a lot of exotic and expensive techniques to reduce mass, the radiators mass about 10 tonnes-- and this doesn't count all the hardware to drive heat to them on the other end.
So, to reject 105W on Earth, I spend about 500g of mass; if I'm as efficient as EATCS, it would be about 15000g of mass.