Filibuster is merely a Senate rule and can be changed at any time.
What it comes down to is how to close a debate. In the House, a debate can be closed by majority vote. In the Senate, it takes 3/5 of the vote to close a debate. That’s all there is to it.
It was a historical accident. Originally, the Senate had a rule to “move the previous question” (i.e., close debate) but it had never been used. The Senate rewrote their rules without a means to close debate back in 1806.
It was a non-issue for almost 25 years until I believe Henry Clay was attempting to close debate on reauthorizing the Second Bank of America. Clay had a majority, but the opposition simply threatened to debate it to death. Even years after that, the filibuster wasn’t really used because the idea is that the Senate could simply change the rule of closure via majority vote, and thus break a filibuster.
In the early 20th century, the rule to close debate was formalized with 2/3 vote. I believe this happened during the Wilson administration. The filibuster only came into a life of its own during the 1930s when Huey Long used it to stop bills he didn’t like. It didn’t take long for Southern Senators to use the filibuster to stop civil rights legislation.
In the 1970s, the rule was changed from 2/3 (66 out of 100) to 3/5 (60 out of 100), but at the same time, the rule to allow the Senate to change the closure rule went from majority to 2/3rds.
In the 21st century, the Republicans threatened the “nuclear option” to get rid of the filibuster when the Democrats were refusing to allow bringing forth some of President W. Bush’s judicial nominees. The Republicans would change the Senate rule that said you needed 2/3rds majority to change the rule on closure via majority vote. Then, change the rule on closure from 3/5 vote to majority vote. Thus ending the filibuster.
In theory, all it will take to get rid of the filibuster is two separate majority votes on rule changes. (I can’t imagine the Republicans successfully objecting to the very same tactic they once threatened to do). The problem is that all the senators know that maybe today the filibuster rule runs against you, but some day you’ll be grateful that its there.