So there's a few problems with that plan:

1. If you leave Narnia, you will eventually forget Narnia. First it's like a dream, then a dream of a dream, and then you just completely forget ever having gone.

2. The same applies in reverse. You will eventually forget Earth and spend your time in Narnia instead.

3. You can't go to Narnia without Aslan taking you there. The Professor, who was infact one of the entities present at the creation of Narnia, tells the Pevensies that they won't be getting back to Narnia through the wardrobe again.

4. Even if you could pass through to Narnia on command, there is a varying degree of time dilation between Narnia and Earth. The entirety of Narnia's 2,555 year existence is compressed into 50 years on Earth, but the first 1000 years of that existence was compressed into the first 40 years of the timeline, and the remaining 1,555 was in that final 10 years. Also, you can spend 10 minutes in Narnia and end up having been gone for weeks on Earth, so the time dilation goes both way and is pretty inconsistent then too.