20-Year Prospect
Well, we're going to go out on a limb and guess that plants will still be around in twenty years. It's a shaky prediction, we know. And as long as people still feel like decorating their homes and outdoor spaces with plants and trees, there should still be nursery workers to help out.
That said, it's hard to predict the "people" portion of retail jobs over the next twenty years. Technology is evolving all the time, and some of the first occupations on the chopping block seem to be those in retail. Agricultural jobs in particular are feeling the effects; they're consolidating operations across the board, and that might mean fewer jobs in the future (source).
If you're interested in working in a retail plant nursery, you'd do well to make sure you pick up the finer points of running a business along the way—you might need to get in the business of selling something else a little further down the road.