The limitation is really the number of charges you have to "calibrate" the mining units , the quality of the tiles you are able to find, and the time you are willing to put in . Based upon what I have seen mining will easily pay for the taxes. Obviously this does not really solve some of the projects that are around like cities, in which case the mining might not pay for the taxes and I could see why they would be upset. On the other hand, the taxes will free up tons of tiles that are not really being used for anything. Regarding why not mine more T2, eventually if everybody mines the T2 instead of the T1 price of T2 will crash and T1 will spike. It will self regulate. But Yes , I agree with you, initially everybody will do the math and most likely do T2 /T3, etc... rather then T1.