I think that what they are saying is the if you sign up before Dec, they will let you in for $10, which they have no present plans to change, for people signed up before Dec.
After Dec, anything could happen, but the implication seems to be that new subscribers will go back to $20 or whatever the price was before all these introductory offers came along (perhaps with a 50% teaser discount for the first year only). Those on the $10 plan will be entitled to remain on the $10 plan unless Adobe changes its mind at some future point.
Just like the banking, cable, or telecoms industries, the incentive for Adobe is to maximise the number of subscribers by giving new subscribers low introductory entry rates while denying those rates to existing subscribers. So you'll get a good rate on your savings account for a year, after which it will plummet. If you don't put your money into a new account, as many people won't be bothered to do, you'll lose out -- inertia selling.