There are four sources of knowledge:
Pratyaksha – Direct perception. You see the Taj Mahal in Agra for yourself.
Anumana – Inference. You see smoke and infer fire.
Upamana – Comparison. You compare living standards of India and the US
Agama – Tradition. Great sages over the ages have declared the same Truth.
Here Krsna is falling back on the tradition laid down by generations of rajarishis, royal sages like Dasharatha, Rama, Janaka and others. They were kingly without but sagely within. They lived in utter luxury yet had perfect control over their mind and senses. They had knowledge of the Self.
Krsna says 5000 years back that over time these values had eroded. This is exactly what has happened in modern times. He addresses Arjuna as Parantapa, scorcher of enemies. The implication to us is – you have the capacity to overcome challenges in life. You cannot let this happen. You must revitalize this great knowledge, and re-install it in the hearts and minds of people and restore humanity to its glory.