Odds of Hanging On
As with any career, the first few years are likely the most difficult. It's not easy to conform to working for a big, stable landscape design company while expressing and utilizing your own distinct style. Smaller firms allow more creative freedom, but are typically not as stable.
Establishing your own firm allows the most freedom, but it's a lot of work and tough to pull off. It takes years to get enough of the reliable clientele, like property management and those who work with real estate, to sustain a landscape design firm. That's why most folks start off at a firm - it gets them some much-needed traction (and contacts).
After a few years, a good landscape designer in a big and stable firm either does rather well, or moves on to establish their own firm. By this time, starting a new firm is still a lot of work, but it's not as risky now that the designer has a good reputation and knows to avoid the forbidden fruit.