Varna is a choice, just like any other profession

Brahmin, Kshatriya etc are Varnas not Castes. There are only 4 Varnas and no Caste anywhere in Vedas.

Varna is a choice, just like any other profession. You can choose if you want to be a Brahmin.

But most people did not because to be a Brahmin meant

1. Not allowed to keep wealth
2. Need to live on Dakshina
3. Can’t do any business
4. Profession very limited
5. Need to follow strict rules
6. More punishment for same crime

A Brahmin who does not perform Sandhyavandanam becomes a Shudra in this birth itself, and a dog in next birth.

Daksha Smrito 2.18-19

Brihadyogi Yajñavalkya Smriti says the same thing.

Any person who does not perform Sandhya becomes a Shudra in this birth itself.

Brahmins, Kshatriyas and Vaishyas are known as Shudras until they get Yajñopavita and start studying Vedas.

Shankh Smriti

Mahabharata says the same thing,

A Shudra can become a Brahmin by attaining good qualities and a Brahmin can become a Shudra similarly.

Even Manusmriti says that a Brahmin becomes Shudra when he keeps the company of bad people.

Manusmriti 4.245

When a Brahmin stops following Vedas, stop following rules, and start eating everything prohibited in Shastras. He / She becomes a Shudra.

King Janashruti (Born in Kshatriya family) was called a Shudra by Rishi Raikva because of his ignorance.

From Chhandogya Upnishad

A Brahmin Becomes a Shudra by selling milk.

Why? because brahmins get cows in donations, using it for earning money is unethical and hence Brahmin becomes Shudra in 3 days.

And there are many more references…