What does that matter? We're talking trifectas here, not supermajorities. The filibuster is a cute remnant of "decorum." It's a vestigial rule which will disappear when too inconvenient. (Fun question with not-so-fun answers: why isn't the filibuster gone already?)
Because I don't think it's vestigial, I think it's serving an important function of governance that never made it into the official rules but is nonetheless necessary as a stabilizing effect. It doesn't have to be the filibuster but something ought to provide the effect. It should be easier to block legislation than to pass it. It wouldn't be a good thing if you could have huge policy swings when a 51-49 becomes 49-51. Being able to, with effort, demand specific pieces of legislation reach a higher bar biases us toward the status quo.
> (Fun question with not-so-fun answers: why isn't the filibuster gone already?)
Because both parties are scared eventually the other party will be back in the majority.