Ok, but that time period is like 100k years.
By your theory people and monkeys are useless terms because over a long enough time period monkeys can potentially turn into people.
You could say the same about strawberries and watermelons. Is it really useless to have a distinction between strawberries and watermelons?
We came up with the terms indica and sativa for the same reasons. They describe two distinctly different types of cannabis. And yes, it is important to have those terms because they describe two different things. They are not less different because put in a different environment and given 100k years they might not be different. By your logic we should only have two words to describe living things. Plants and animals. Everything else is a meaningless distinction. But personally I want those labels so I can make sure I'm petting a house cat and not a wild tiger. When I smoke bud to go to bed I want to make sure I'm smoking a indica and not a sativa.
Because those terms are useless in describing the effects of the cannabis you're about to ingest.
Well what you appear to be describing is evolution. And there is plenty of scientific proof of it. But I don't think it happens as quickly as you think it happens. If you take a south American sativa and grow it in India, breading it only into itself, it isn't going to turn into an indica after a few grows. That wouldn't even happen in your great great great grandchildren's lifetime. And even if it does adapt to it's environment over thousands of years, it's not necessarily going to make the same adaptions indicas made. Evolution doesn't work that way as far as we know.