Snakes do recover from RI if treated properly, that means early as possible and long enough by a reputable herp vet.
Sure some animals with weaker immune system may not recover or may develop one RI after another but it's no a common occurrence, and each case should be evaluated before deciding to euthanize.
Honestly I am not surprised by what you were told and I have heard even worse before (but that would be another debate) , I guess for some people it's just value of the animal vs cost of treatment.......sad but true.