Naturally, the development of pediatric cardiology, thoracic surgery and pediatric intensive care have been essential for this dramatic improvement.
The improved situation for infants with congenital heart disease is quite similar and parallel to improved outcomes for preterm infants. Only a few decades ago, the majority of today's survivors would have had a poor prognosis.
With this growing generation of "new survivors" comes new challenges. In neonatal and adult medicine we know little about aging individuals born preterm, and I believe the same applies to aging individuals born with congenital heart disease.
Knowles RL, Bull C, Wren C, & Dezateux C (2012). Mortality with congenital heart defects in England and Wales, 1959-2009: exploring technological change through period and birth cohort analysis. Archives of disease in childhood, 97 (10), 861-5 PMID: 22753769