First off, the "gunk" you describe is an anti rust inhibitor and lubricator that is installed at factory. This "gunk" helps keep the internal parts on the water pump etc lubricated. THIS IS NOT a by-product of DEXCOOL. This substance is prevalent on the dipstick and gives the appearance of sludge or dirt. Many assume its sludge and will do harm. On the contrary it is there to help prolong the parts and keep rust deposits at bay.
Many panic and flush the system then complain their radiators failed prematurely. All they did was remove the product the factory installed to prolong the life of the radiator and components.
Second, DEXCOOL is "supposed" to give more longevity and retain its properties longer than anti-freeze. If one reads the manual it states this. As long as all the DEXCOOL is removed so it does not mix with anti-freeze which will do harm to the coolant system before replacing with anti-freeze, no harm no foul.
Why one would remove a what is supposed to be superior product that reduces costs and increases longevity and provide a better alternative to anti-freeze that costs the same as anti-freeze with a less inferior product :confused
Many don't understand or are more comfortable with the old.