That's not actually correct. Some of our cells have high turnover, but many of our cells surprisingly don't. The obvious example is nerve cells, which stop replicating at some point in our development and we have the same set for the rest of our lives (probably with caveats as always). A more surprising example is skeletal muscle cells, which except for specific circumstances, don't divide. We pretty much have the same number of muscle cells our whole lives, they just grow and shrink in response to training or lack thereof, again with caveats. Another example is fat cells. Most people also don't get new fat cells, but some people can make new fat cells, so it's even complicated for that one example. So to tie things up, you don't actually replace all of your cells every 7 years, and in fact the cells that are some of the most important to your survival generally never get replaced.