You seem to be taking the view that because something is possible for the Govt to do and because there is nothing that you as an individual can do to stop them, then that is what they will do - irrespective of whether it makes sense.
By the same token they could apply a tax on all blue cows, or half eaten donuts.
Can you explain the logic and practicality of removing TRS from 1 specific mortage product, which itself contains varying levels - from maybe 0.35% to 3.5%?
If you mean a mortgage on an investment property then you will not be affected by any change to TRS - you have already been affected due to the 75% limit on claiming tax relief on the mortgage interest.