well they don't so why worry about it? that's like asking what if pigs could fly!?
Answer 2: I guess it depends on what you mean by "walk." To me walking implies that they have legs. Snails don't have legs, so they glide rather than walk. In order for a plant to have legs, it would have to have the genetics that underpin everything a leg is, and everything that makes it more. Which is fine, because plants can have huge genomes.
More practically, if plants could literally walk, I would expect that they wouldn't need to put so much effort into creating flowers and complex fruits. That would be a problem for humans, because all we would have to eat is leaves, mushrooms, microscopic organisms, algae, and animals. And if you had to chase a lettuce plant around just for a salad, that would be a real pain. I guess eating seaweed would become more popular.
The ecosystem of a place would be changing every now and then