their answer was clear, no there will be no level 7, so yes it is stopped on level 6, and if they did plan for that, there would be more resources types.
Also, titanium does not look like a "building material" anyway, the answer like that really sounds like a final answer, because if you read other answers, if it is something possible, it is clear it something maybe will come.
You maybe say there is "coins" ok, you can say it the cost of new furniture and equipment :P
Anyway there is a lot of improvement versions of buildings, and doing level 7 will just make things really hit more than peak, and make trade with things cheaper if you worry about cheap titanium, level 7 like what you suggest will increase the price of titanium but will drop the rest of prices badly, the resources sinking now is already not enough, level 7 will make it worst.