How the varnashrama system is sanatana

The Vedas describe how to divide the human race into four divisions according to quality and working capacity. This is a very scientific system, and it is also sanatana, for no one can trace out its history and it has no dissolution. No one can stop the system of varna and asrama, or the castes and divisions. For example, whether or not one accepts the name brahmana, there is a class in society which is known as the intelligent class and which is interested in spiritual understanding and philosophy. Similarly, there is a class of men who are interested in administration and in ruling others. In the Vedic system these martially spirited men are called ksatriyas. Similarly, everywhere there is a class of men who are interested in economic development, business, industry and money-making; they are called vaisyas. And there is another class who are neither intelligent nor martially spirited nor endowed with the capacity for economic development but who simply can serve others. They are called sudras, or the laborer class. This system is sanatana–it comes from time immemorial, and it will continue in the same way. There is no power in the world which can stop it. Therefore, since this sanatana-dharma system is eternal, one can elevate himself to the highest standard of spiritual life by following the Vedic principles.

Srimad Bhagavatam 4.2.31 purport