Keeping the 767 line open with tankers will do nothing to help product developement
I'm not so sure. If the manufacturer makes even a 1% profit - then that can contribute towards R&D of other products. It may not be a direct subsidy but if it helps the company retain staff in work, then it has political benefits. If, when the (say) tankers are finished, the staff then go on to work on the new project - it might be thought of as subsidy. If Boeing had to close a production line and cut the staff, they would not be in such a good positon when the new a/c was ready and they needed production staff.
However, the argument is a pointless one. EVERY govt will support it's own companies whenever it can irrespective of the area/type. Building weapons or aircraft, or constructing a new national building, they will try and help their own. Many would argue that is a prime and valid function of govt.