Adobe is just doing what any corporation does - charge as much as they can for their product, and try to lock in "recurring revenue". The problem in this case is lack of competition. Microsoft was in this position for a long time, before Apple became a signifcant threat, and that only happened because the market changed signficantly (focusing on mobile platforms for which Microsoft was unprepared).
I hate the idea of a monthly bill, but the fact that they'll be able to raise it anytime they want bothers me more. And what bothers me even more than that, is that they'll no longer need significant enhancements to the product at regular intervals. What's to stop them from cutting their whole R&D group? The money would just keep rolling in for years...