Of course the Triple Crown has always been a big deal - Secretariat was on the cover of Time magazine, for heaven’s sakes, and many of the old greats are legends in their own right among people who know nothing about horses or racing. I have many, many friends who are neither horse people nor avid racing fans, and they know Secretariat, Seattle Slew, etc.
The Triple Crown has ALWAYS been a huge event, even before the days of ESPN and TGV. The $5 million dollar bonus merely sweetened the pot - just like all those other bonuses that are offered for winning X, Y, and Z races together.