I know, I late for this question from the developers, but, I have an other question.

Why the travian system have to be a close system?

Just think about it. Every mine is infinite (theoritically) in the main village, but the warehouse and grannaries capabilities are not, so there are limits for it.
Why the whole system is an open thing, but if you want to build a very high level building, or mine, you have to pay for it dearly.