something I do not understand is this
A: Yes, you can! It's an important part of your mining strategy, but the value of your building depreciates every day. Wear and tear wreak havoc on them the longer you earn Coins with your buildings.
What does it mean, the value of the building until it goes down, it will go down to zero? and stop giving me coins? or will it disappear?
There is a floor in depreciation, but this matters only if you plan to sell your building. Your buildings will give you coins as long as you keep your efficiency and your MP is over 34 (33,7 actually).
Some leaks about the coming update will change a bit the things as it seems your levels will "expire" with time and you'll have to reinstate them by using gems, but this will be explained in full when the update will be rolled.
For now, keep efficiency the highest you can and your buildings will produce coins. Happy earning