If you're switching jobs just for more money and don't get any useful experience or skills along they way, effectively coasting along and becoming a junior with X years of experience, then yes you can be replaced.
However, if you're smart and a little bit selective, making sure to only job hop to gigs that provide some challenge and growth opportunities, not just going with every recruiter that promises you more $$$, then the skills you'll develop and pick up should guarantee that no junior can easily replace you.