The quote is that the tree is “refreshed” with blood, which is an important distinction. Also, Jefferson wrote it after the founding of the US - he understood that our democracy is not an exception to this cycle.

Yes, if we all did our civic duty not just to vote, but to actually inform ourselves about the choices, we’d be able to maintain democracy potentially indefinitely, but the reality is that a huge portion of people are complacent, and won’t take even the simplest of actions until they’re forced to. So, democracy degrades slowly as it’s desperately propped up by the few who understand its importance until it finally fails enough to start really affecting the people who “aren’t really into politics,” by which point its too late to use sweat instead of blood.

We water the tree with the sweat of the few, but when that inevitably isn’t enough and it starts to wither, we refresh it with blood of the many.