well, I personally have no first hand knowledge of why Thune is doing what he does,. But, in my experience, I have discovered that in Congress , both house and Senate, there is a hesitation to bring something to the floor if it doesn't have the votes to succeed. Both sides have whips whose job it is to count votes. So thune knows where these stand success wise.
The filibuster makes both sides work together, compromise, and while the result might not be the best possible, it's the best that people with sometimes totally opposite viewpoints will support. Historically, there have been plenty of major items of legislation passed when the filibuster was intact.
Seems to me we could do with a little compromise politically right now.