But that's beside the point. You are mistaking the discriminatory nature of 'caste' with just the different nature of 'varna's. There was fluidity among varnas long before the British arrived and also inter-varna and inter-jati marriages.
> Tamil Brahmins have completely different gene line from say Chitpavanas and yet both are seen as Brahmins and they will quickly form a good relationship compared to say a Chitpavana and Koli (fisherman)
Where are you getting such information from? As a person who has lived in India for several decades, I can tell you this is not true. Nobody asks which varna or jati you are from, or no one even mentions these words at least in urban parts.
> Even if it was a British construct it is still the failure of Indian society to continue with it...
We never had a discriminatory system in the first place, and the only thing that remains today is the damage done by our various colonizers. India's Prime Minister Sri Narendra Modi is himself part of an OBC (other backward classes) community (Modh Ganchi) and has enormous support from Indians from all backgrounds, India's President Smt. Draupadi Murmu is from a ST (Schedule Tribe) community called Santhali. The chief of the drafting committee of India's constitution is a Dalit, as you may know, Dr. Ambedkar, who was in strong opposition of the Brahmanical patriarchy himself. Yet he was given such an important role, in a country where ~80% of the people are Hindus.
Chatrapati Shivaji's father was a Jagirdaar serving under the Nizam, so their family was pretty well off. You are right that he was from a family of land-tillers which is considered to be of a Shudhra varna. However, he employed people of all varnas in his operations.
> Once a person or tribe builds social capital they automatically transition into upper-caste. Those who do not, remain at the bottom.
Now that's just... head cannon. President Draupadi Murmu being from a ST does not make any effort to hide the fact. Getting to the post of President has not changed that. Similarly, Dr. Ambedkar's achievements got him far, but he still was a Dalit. Same with Narendra Modi.