Money.
I'm going to pull the old motor apart to see exactly what happened but I think you are on the money. This is my moms truck and she doesn't know any better so... It's going to be a lil project locating a used motor and swapping them out.
i'm a gm dealership shop foreman, i see it all the time, even had to replace my own motor in my 99 s10 blazer last month cause of this shit! when you do the motor, have the radiator rodded and cleaned. only way to totally get rid of the stuff is to replace everything that has to do with coolant. put good ole green back in
ps. i found a decent motor reman place and bought my short block for 350bucks, fresh 40 over. might check into doin that and save yourself a buck. i had my heads worked, replaced all hoses, replace the water pump, replaced the motor, had the radiator rodded and cleaned. i still see alittle mud floating around, which is coming outta the heater core its gonna leak over the winter i'm sure i think i got out for under 1k doin that way.