I would expect Disney to be willing to operate at a loss for years on Disney+, in order to catch up some on subscribers. Investors will want to see sales & subscriber growth more than profit at this point, from that service. They're not expecting profit anytime soon. A failure on gaining subscribers will be a big hit to the stock however, it'll indicate a permanent subservience to Netflix in streaming. If they generate large subscriber gains, the market will reward them for it considerably and forward bet on eventual profits.
Disney has $13.x billion in operating income, they can easily afford to try to tip over Netflix's money burning machine and try to hurt them financially. The $12.99 figure puts a further cap on the pricing power that Netflix has at this point, which is also a cap on their ability to fix their severe cash burn problem. Disney can play a long game and watch Netflix continue to drown in debt interest costs.