Seems a lot of people are just repeating "Pokemon will keep going it's a cash cow franchise."
And while that's true, I'd like to add a bit more to the conversation.
Something a user on another forum I go to, Erik the Appreciator, pointed out that not only are there still many RL species that have yet to be turned into Pokemon, but also that they have already used multiple species more than once. Think of all the dog and cat pokemon. It's kind of like how there's different breeds of cats and dogs, but they're all the same species (although in Pokemon they are different species but that's beside the point). There are several different caterpillar mons. There are Seel, and then two gens later, spheal. Both of them are seals, but they feel unique. Thus, the games have a HUGE potential to keep going and adding more monsters.
Next, let's look at the comment made about seven Pokemon regions for seven continents. *cough*regionsnotcontinents*cough*
No, seriously, all of the MAIN SERIES regions have been based on parts of Japan. They haven't even used all of Japan, boys and girls, much less an entire continent. Another reason it can keep going.
For those of you who say there's not much more they can add in the way of gameplay enhancements and innovations, you are wrong. They keep adding more, and there's a lot more yet they can still do. And knowing Nintendo, they probably have a lot of ideas I haven't thought of.
The series itself is highly adaptable. Look at Colosseum, for instance, or the Ranger or Mystery Dungeon series. It has the ability to change with the times.
TL;DR: Pokemon seems to have been specifically designed to endure.
