Although there are many different routes to decrease population growth, if they are not secure enough to last for about 40 years many problems may increase and become worst than now.
Notes: I would use "ways" or "methods", not routes.
Secure is not the best word to describe a method such as this.
I suggested "could", not "may". May suggests that something is allowed to happen, not that it might happen. It's a common confusion.
In your original sentence, you should have used "worse", not "worst" - worst is the superlative, but if you are going to say "than now" after it, you need the comparative.