That is 100% going to happen, it's already prefaced in the emails. They didn't guarantee locked in at $15, they guaranteed locked in at 40% off the retail price, whatever that price is when it changes.
Think about it, they are releasing like 10 new plugins this summer alone. By this time next year they'll be advertising that the subscription carries 120+ plugins, so the price has to go up.
I think this thread did come up with a reasonable solution. From reading everyone's opinions, seems that the "you pick X" model would solve most of the problems. If people can pick 20 plugins they want it helps reduce the cost, addresses the fact that nobody needs the entire catalogue, and lets loyal customers pick plugins they don't yet own.
Think about this, how many people here would object to:
Pick 10 plugins $5 /m
Pick 20 plugins $9 /m
Entire collection $19 /m