If Sinema loses she will likely be replaced by a Republican. If Manchin lost a Republican would be guaranteed to take that seat. The problem isn’t the filibuster, (needing 60 votes to pass legislation is a good idea to slow things down, make sure there’s general consensus, and stability) it’s the primary structure that puts out polarizing politicians that think too ideologically and don’t want to work with the other side. Elections shouldn’t be between 2 people who disagree on every major issue.