If AC expects travellers who used to buy the "old" cheaper Tango Plus TPAC (which practically is today's Tango fare) to buy the "new" more expensive Tango Plus TPAC, it has to offer a little more by effectively creating a "premium economy" section, similar to their previous Connaisseur service. Start by taking out a row or two to increase the seat pitch.
The fare difference between AC's Tango Plus and some other airlines' real premium economy is so little that the Tango Plus price is simply uncompetitive.