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 $$$, then the skills you'll develop and pick up should guarantee that no junior can easily replace you.
If you're switching jobs just for more money and don't get any useful experience or skills along they way, effectively coasting along and becoming a junior with X years of experience, then yes you can be replaced.
However, if you're smart and a little bit selective, making sure to only job hop to gigs that provide some challenge and growth opportunities, not just going with every recruiter that promises you more $$$, then the skills you'll develop and pick up should guarantee that no junior can easily replace you.
My $0.02.